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Sauna Walls: Insulate With Vapor Barrier or Let Them Breathe?

If our sauna walls could talk, here’s what they’d say: “seal off the hot room, don’t let this moisture get in here!”

There are a couple schools of thought regarding sauna construction and sauna walls:

  1. Insulate, foil vapor barrier and tape the seams well.
  2. Don’t do anything, let the walls breathe.

Those that prescribe to #1 (insulating and sealing our sauna walls with vapor barrier) believe that we need to keep heat and moisture away from our wall cavities (and nowhere near the inside).

Those that prescribe to #2 (letting their sauna walls breathe) believe that heat and moisture want to escape and permeate through the building (and out the other side).

This casual sauna builder is very much a proponent of #1. Why? 3 reasons:

  1. Log sauna goodness: Most agree that traditional log saunas feel the best. Sitting on the sauna bench, solid wood logs give the sauna bather a more solid, dense, soft heat kind of feel. This is the best way I can describe the affinity towards solid wall saunas. But the point is that a well insulated stick frame sauna plus a well sealed vapor barrier provides a comparable dense sauna wall, replicating (?) a solid wall sauna building.
  2. No moisture leaking: Warm wet air will always rush to colder dry air. This is why freezers (before frost free technology) used to always get ice build up. “Close the door!” Warm wet air will always rush to colder dry air. This is an issue when we don’t seal off our hot rooms. Warm wet air from our hot rooms will rush to wall cavities and settle there as condensation against cooler wood paneling. Damp, wet, rot. Not cool.
  3. Thermal containment: A well insulated hot room with a well sealed vapor barrier holds in moisture and holds in heat. “But our sauna walls aren’t insulated and we can get it up to 100 degrees Celsius (212 degrees Fahrenheit)” That’s great, but could that be like saying that you can eventually cook a pizza with your oven door open?

“Don’t use vapor barrier, let your walls breathe.” Hogwash, I say. We build our saunas once.

As you look at the wall cavities of your sauna building, think about how little insulation costs, and how easy insulation is to install. Think about how easy it is to foil vapor barrier your hot room and how foil bubble wrap is sauna building secret #5. And think about how this casual sauna builder has repaired saunas that were insulated properly and sealed off properly 30 years before, and how after opening up walls between sauna hot room and outside, this casual sauna builder sees no sign of any moisture, mold, decay whatsoever. And think about how this same casual sauna builder has opened up walls that were not sealed properly and have found studs and bottom plates that were rotten from moisture hanging on like a wet rag to an armchair.

Can I Use Plastic Foam to Insulate My Sauna?

Yes, you can use polyurethane, polystyrene, or polyisocyanurate foam to insulate your sauna, but should you?

Guest post series continues. Please welcome Nick who is an urban free range organic egg farmer and kombucha maker from North Minneapolis, MN USA. He is millennial aged and newly parented. Nick is currently building his own mobile sauna. He wants to do his best to provide a safe, healthy sauna experience for his young family. After 14 phone calls, 11 texts, and 7 email exchanges, I asked him to kindly write up his discoveries, which he was happy to do, and we agreed to share this for all saunatimes readers.

Enter Nick

While you will find much debate, there is often very little variation in the types of insulation that are recommended for sauna building. For decades, fiberglass batts have been the workhorse in the United States. It’s cheap, naturally noncombustible, and you don’t have to worry too much about moisture if you properly install a vapor barrier (a big IF).

In Europe, mineral wool is the insulation of choice for many sauna builders with its extreme heat, moisture, and mold resistant properties. Historically, mineral wool has been harder to procure in the US, but nowadays it’s generally widely available with Rockwool and Thermafiber being the primary manufacturers.

With two established and time-tested options, why would we ever consider using an alternative? Especially when that alternative is made of plastic, uses ozone depleting blowing agents, and has the potential to off gas dangerous chemicals when exposed to high heat. Well, those of us in search of lämpömassa know that saunas take endless forms and can be built in a variety of places. Fiberglass or mineral wool insulation shine in your standard sauna build: the backyard shed, converted garage, or bathroom sauna. But we also build saunas in damp concrete basements where löyly isn’t the only moisture to consider when making your insulation choice. There are even greater considerations with the increasingly popular mobile sauna (or peräkärrysauna) where one might have to account for variables such thinner walls, non-permeable metal exteriors, and the potential for your peräkärrysauna to bounce down the highway at 80 miles per hour.

Depending on your circumstances (and constraints), you may find yourself Googling, “Can I use foam board to insulate my sauna?” or, in my case, “Is spray foam safe for sauna?” And what you’ll find is a whole lot of opinions, but very little research when it comes to using polyurethane, polystyrene or polyisocyanurate foam for sauna insulation. Expectedly, all the “experts” on the internet forums scream “Don’t ever use foam for a sauna! It will off gas horribly!”

While I generally agree with that first statement (since I am a millennial hippie and think limiting plastics in our life is a good thing and I hope you do too), I didn’t find very much actual research to back up health concerns related to VOCs or off gassing. That doesn’t necessarily mean using foam is safe, especially if you don’t follow maximum service temperature manufacturer guidelines. All materials have some degree of off gassing (what do you think that smell is from your T&G cedar?), and since several of the materials used to make foam insulation are known to have health impacts on humans and our planet, it’s fair to be concerned about what we might be breathing in our saunas. Specifically, all foam insulations use potentially hazardous blowing agents and flame retardants.

Since you’re stressing out about if you should use foam for insulation, let’s go through the various, widely used foam insulation products to help you determine their appropriateness, and my (somewhat arbitrary) off gassing risk rating for using them in sauna building:

EPS – Expanded Polystyrene Foam

Expanded Polystyrene - EPS - Thermal Insulation

This is your standard styrofoam insulation. It’s cheap, lightweight, and is about 98 percent air. Think of your styrofoam plastic cup. It’s produced from styrene, which forms the cellular structure, and uses pentane as the blowing agent. Styrene is listed as a “probable carcinogen” by the World Health Organization. Despite that, there isn’t any evidence that it off gasses under normal conditions. The biggest limitation for using EPS in sauna applications is that the material has very low maximum service temperature of 165°F. Historically, there have also been concerns about the flame retardants used in EPS.

  • R-Value at 1 inch: R-3.85
  • Max Service Temp: 165°F
  • Off gassing risk at sauna temps: High

XPS – Extruded Polystyrene Foam Panels

XPS Foam Sheet - Rubber sheet|Sponge Foam rubber sheet- China ...

XPS foam board is made with similar materials to EPS (styrene) but is manufactured using a different process and different blowing agents. The result is a more rigid and vapor resistant product. Traditionally, XPS uses fluorocarbons as blowing agents, which can negatively impact the ozone. However, many manufacturers are planning to phase out problematic blowing agents in the coming years. Additionally, toxic flame retardants in these materials can be released as dust, though many manufacturers are starting to use less toxic options. Similar to EPS, XPS probably isn’t a great option for sauna due to a maximum service temp of 165 F. Though you could make the case for using it if it was buried behind some fiberglass or mineral wool in a basement build.

  • R-Value at 1 inch: R-5 (this is somewhat debated due to Thermal Drift)
  • Max Service Temp: 165°F
  • Off gassing risk at sauna temps: High

Polyisocyanurate Foam Board

The difference between Polyiso, EPS & XPS Foam Insulation ...

Polyiso is a foam board product that often comes with foil facing and is used primarily for continuous insulation as external sheathing or in roofing applications. However, it can be used internally and between studs, and it is a favorite insulation for van conversions. It’s slightly more delicate than XPS and there are some concerns about it soaking up moisture, however the foil facers provide some structure and vapor protection. Polyiso uses CO2 and pentane as the blowing agent and off gassing from diffusion or rupture are not considered a health concern. You probably breath more pentane on a trip to fill up your car at a gas station then you ever could from trace pentane off gassing from insulation.

This bigger concern with polyiso is that it often contains TCPP as a flame retardant, which is considered toxic and can have serious health impacts on humans. The flame retardants will be released as dust over time in the form of dust as opposed to gas. If they become dust borne, the main route of exposure is hand to mouth. There are several companies producing polyiso using halogen-free flame retardants, which is chemically bonded to the polyiso polymer so there is no flame retardant that can leach out. GAF EnergyGaurd is example of a halogen-free product. Availability of halogen-free polyiso may be a barrier, since I haven’t been able to find anywhere to purchase it.

Polyiso is rated with a maximum service temperature of 250°F, which I can’t imagine it ever getting to if used within the walls of a sauna. The fact that it also has aluminum facers provides the additional benefit of a built-in vapor and radiant barrier (depending how it’s installed).

  • R- Value at 1 inch: R-6 (this is somewhat debated and decreases significantly in colder climates due to Thermal Drift)
  • Max Service Temp: 250°F
  • Off gassing risk at sauna temps: Moderate

Polyurethane Spray Foam

Spray foam is a two-part polyurethane that is applied as a liquid but expands into a foam as it dries. The first part of the mix contains methylene diphenyl diisocyanate (MDI) and the second part of the mix contains amines, glycols, and flame retardants. While the hazards of MDI are well documented, when these two components combine they form a completely different material (A + B = C) that no longer has the same danger of the original raw materials (aside from the off gassing of flame retardants). Once properly cured, manufacturers claim that spray foam is non-toxic.

However, spray foam is probably one of the most controversial foam products I dug into during my Googling. Unlike foam board, where the chemical reactions to manufacture the insulation take place in heavily controlled manufacturing environment, spray foam is applied “out in the field.” This means the chemical reactions are happening under a variety of temperatures and conditions. You can find horror stories across the internet about spray foam jobs gone wrong which result in indefinite off gassing of the chemicals that are mixed to produce the foam. While these incidents do occur, which can have long lasting health impacts on those who are unfortunate enough to experience it, the rate at which it happens is likely very minimal. There is a lot spray foam being installed in the United States. They key is to hire someone who knows what they are doing.

Despite many assurances that spray foam is safe, as long as it cures properly, I couldn’t find much research about it generally. In fact, the EPA website states, “The potential for off-gassing of volatile chemicals from spray polyurethane foam is not fully understood and is an area where more research is needed.” So, I reached out to a researcher for one of the only research studies I could find on emissions from spray polyurethane foam (see Characterization of Emissions from Spray Polyurethane Foam here) and this is what I learned:

  • In general, emissions from these materials increase exponentially with temperature increases. There is not data on the rate of the increase or how much is emitted at higher temps.
  • There are many claims made by manufacturers regarding emissions that are not verified independently. So even though a manufacturer claims a product is “low-VOC” and is rated for a maximum service temperature, it does not necessarily mean that there aren’t emissions from a product.
  • There is no data on the migration of emission from spray foam through wall components, such as vapor barriers.
  • There is only one standard test method for testing emissions from spray foams (https://www.astm.org/Standards/D8142.htm). It costs several thousands of dollars to run.

While the statements above apply to spray foam, they are likely relevant to some other types of foam insulation as well since they contain similar materials. One very unique advantage of spray foam (closed-cell specifically) for sauna application is that it provides very high R-value and is an excellent vapor barrier. In theory, this makes it ideal for some mobile sauna applications. However, the maximum service temperature for spray foam can vary greatly from 180°F up to 250°F, so it’s important to pick your product (and the person applying it) very carefully.

  • R-Value at 1 inch: R-6.6 (closed-cell)
  • Max Service Temp: 180°F – 250°F
  • Off gassing risk at sauna temps: Moderate

In Closing

Despite many weeks of internet scrolling, I concluded, rather unsatisfyingly, that there is not a clear answer when it comes to using foam to insulate sauna. Each of us has to make the choice we are comfortable making with the information that is available. Sounds a lot like most areas of life, eh? Another life lesson from sauna building. Oh, and make sure you do a kick ass job with the vapor barrier. Tape those seams!

Notes

We were able to visit with John Elverum, Technical Service Specialist at Johns Manville, makers of the AP Foil Faced Polyiso. John directed us to the spec sheet for this material. Included therein, under table 2, is the service temperature rating. (250°F, 121°C). Per John, temperatures greater than 250°F, and polyiso (AP Foil Faced) is vulnerable to shrinkage, and changing shape. Link to Johns Manville Polyiso product sheet here.

Those suspect of Poly-iso can be supported by The Sauna Twins at Finnmark Sauna. Please listen to our Sauna Talk here. Flammability is indeed an issue of consideration. Is there a magic product for insulating our saunas? Do these choices of material affect indoor air emissions? Atlas EnergyShield and GAF EnergyGaurd offer halogen-free polyiso. However, it’s hard to source.

What the Experts Say

I recently had the pleasure to sit down with Todd, who has more experience with insulation of walls and attics, bypass sealing, blower door testing, than anyone I have ever met. Todd is a practical thinker. He is not driven towards one system vs. another, but views each project independently. Oh, and Todd has 25 years experience with enjoying saunas.

There are a couple different techniques:

  1. Closed cell. – expensive. $600-700 kits at Home Depot. Blow in. Expensive.
  2. Pink rigid foam board – cut to fit joist cavities and fill the gaps with Great Stuff. No foil vapor barrier needed.

How About Batting and then Foil?

This works, but it’s not as good as closed cell. Why? Batting and moisture are not friends. Even with insulating our walls and foil, there is a chance for moisture permeation.

What About Just Letting your Walls Breathe?

Todd is not a fan. We want to make our hot rooms consistent and efficient. Everything will work better if we are containing things in the area where you want to be. Less fuel.

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219 Comments

219 thoughts on “Sauna Insulation”

  1. And another thing about “trapping moisture in our joist cavities.” Sure, this can happen if we Tyvec house wrap the outside and vapor barrier the inside but only IF we have moisture permeation because of the failing of one of these barriers. Build our saunas right and we get ZERO moisture infultration.

    And when we insulate our saunas well, we do not allow any cold air to come in contact with warm moist air, so there’s no way moisture will get into our walls.

    This is why we vent our hot rooms with a simple vent., or practice the “bake and breathe” method of drying out our saunas between sessions. https://www.saunatimes.com/sauna-culture/tips/bake-and-breathe-the-best-way-to-keep-our-saunas-germ-free-and-clean/

  2. I have not yet constructed my sauna. I want to do it right the first time.
    I agree with the bubble wrap vapor barrier, it is essential in a cold climate.
    Insulation is another matter.
    Insulation retards heat transfer but does not retain heat. The heat retention is only held in the hot room air and the cedar planking. The insulation has no thermal mass. A log wall once heated up keeps the interior space of the sauna warm and reduces temperature fluctuations. The thermal mass of the log wall radiates heat back into the sauna. Insulation can not do that.
    That said, my plan is to build the hot room within a larger sauna structure, a room within a room. 2×6 hot room walls filled with packed coarse sawdust from a local sawmill. The heat loss will radiate into the larger interior space occupied by the dressing room. The exterior walls of the larger structure will be insulated with standard insulation.
    Is this more work, yes. Is this more expensive and complicated, you betcha.
    Is it worth the effort, I will find out and let you know.

  3. A quick comment – we built our sauna (2×4 stick built, foil vapour barrier, western red cedar), and put Tyvek on the outside, but as we ran out of reasonable weather, couldn’t get the exterior siding on. We used the sauna all winter – no sign whatsoever of ice build up anywhere on the outside…. I agree with #1.

  4. my outdoor unit has tyvek on the exterior and foil on the hot room interior, no complaints. changing room has faced insulation on the exterior walls but unfaced insulation on the interior shared wall with the hot room. do not use faced insulation in that wall as the foil on the hot room side and kraft paper on the changing room side will create a ‘vapor barrier sandwich’ which will trap any moisture that gets in that wall. moisture finding its way into other walls is not a big deal since it has a way to breath out (through the house wrap).

  5. Throwing in my two cents here, I built my sauna pretty much like Miller describes and have had excellent results. This was based on lots of research from sauna sites, community comments, and just what seemed to make sense from both from a heat retention and sealing point of view.

    From the inside out:

    1) cedar T&G planks (9/16″ thickness)
    2) foil vapor barrier ($40 from The Sauna Place, didn’t use foil bubble wrap due to concerns about outgassing, perhaps being paranoid)
    3) All seams and the holes I clumsily poked sealed with 2″ aluminum tape
    4) R13 paper-faced fiberglass insulation, paper side toward the hot room (R19 in the ceiling made from 2×6’s)
    5) 1/2″ exterior grade plywood sheathing
    6) Tyvek house wrap
    7) Hardiplank siding with flashing used above windows, doors, etc.

    We’ve had zero problems with moisture and the insulation really works well from both a heat retention and sound perspective. The sound part is nice for situations like when the neighbor decides it’s time to mow the lawn right in the middle of a sauna session.

    As to heat retention I have a thermometer in the changing room and I’ve never seen the changing room temp increase more than a degree or so despite the fact the hot room went from 60 deg to 180 deg. This shows just how well it works for the interior wall, the exterior are no different.

    I think sometimes we can overthink things, I know I’m certainly prone to it. The outside of the sauna is no different from any other structure so using tried and true methods makes sense. For the inside of the sauna we just need to keep the water out of the walls and the heat inside.

    If people are worried about moisture I would suggest installing an exhaust fan in an upper corner of the hot room. We used a standard bathroom fan with a nice cedar grill. It’s wired in to the electric heater control so when the heater is turned off it runs for 45 minutes then shuts itself off automatically. The fan has had no problem with heat but it in case of problems it can be easily replaced by removing the cedar grill and they are really cheap. It works great for us, I suppose it could cause drafting problems if using a wood burning stove.

  6. i used unfaced insulation in the hot room since the foil already acts as a vapor barrier. using faced insulation results in a gap between the foil and insulation facing which could trap moisture with no easy way to get out. likely a small concern but something to keep in mind.

    for the exterior, i used cedar bevel siding with stain on both sides of the siding and on the cut ends. i took it a step further and nailed 1/4″ latts vertically at the studs, over the sheathing/tyvek. siding is attached to the latts so there is an air channel between the tyvek and the siding. the bottom edge has screen material to keep bugs out and the top extends into the attic, which includes a ridge vent on the roof. so any moisture that makes its way behind the siding is dried out by the ‘chimney effect’ of air moving behind the siding. it doesn’t do anything for moisture concerns inside the sauna but should extend the life of the siding.

  7. Starting my sauna build in earnest and have a question regarding vapor barriers. I am converting an existing building that is 2×4 framed with a painted T1-11 siding. This acts as both the sheathing as well as the exterior siding which means that I have no vapor barrier. Will this be an issue with insulation and interior foil bubble wrap? Should I use a faced insulation and if so, which side should I put the paper?

    My gut says to use unfaced paper since this interior foil will stop moisture from moving into the walls and the lack of house wrap will let the walls breather to the outside. But truly I have no idea. Any help?

  8. Your gut is spot on. unfaced R13 between the 2×4 joist cavities. Then foil bubble wrap, enveloping the hot room completely. Tape seams and around windows and doors well. Foil tape.

  9. Shaun: A fair outline to your situation. If it makes you feel any better, Northern Minnesota can get Northern Canada winter temps, so I am well familiar with what you’re up for and against. Insulate all walls, and insulate them well. Consider more insulation in the ceiling. Yes, foil seal everything. Venting: easy. As you build your hot room door, leave a 3/4″ crack along the bottom. This will allow air to be drawn into your hot room, to feed your stove and vent your hot room from an intake perspective. (this is well detailed in my ebook, look to the right —>).

    Now, as far as venting moist warm air from your hot room, I suggest a small dryer vent opposite wall from your stove about eye height. Most of the time, this vent will be closed. But those rare times when your hot room is hot and steamy as hell, and you don’t want that awesomeness in your changing area, you can open that vent and leave hot room door closed.

    Consider that warm moist air wants to go where cold dry air is. It’s just the way it is. So, on a f**ing cold Northern Canada winter’s eve, you’re going to have some really crazy dynamics going on. Open the hot room door: hot wet air will rush out your hot room, and settle into your changing room. Open the changing room door: hot wet air will rush out your changing room, into the cold Northern Canada winter’s eve. All this is good stuff. No better vibe than hanging out in the changing room, barely able to see through to the opposite wall, breathing deep with an aura akin to a Hawaii mountain steamy morning.

    With well insulated (and sealed) walls, we can control this aura, creating the warmth and humidity that is therapeutic to our minds, bodies and our spirits (on a cold winter’s night). Oh, and check here: https://www.saunatimes.com/sauna-culture/tips/bake-and-breathe-the-best-way-to-keep-our-saunas-germ-free-and-clean/

  10. Hello, I am just about to start building myself a sauna and I have a few questions regarding insulation, sealing and ventilation that I cant seem to find an adequate answer to online.

    I am hoping to be able to use my sauna all winter long and I live in northern Canada where it regularly gets to -40C. My concerns about sealing up my sauna revolve around proper ventilation. This will be an outdoor sauna with a wood stove in it for heat. The wood stove will have its own cold air intake. If I seal the hot room up all nice and tight then I will have to have an air intake and outtake and I’m worried that the very cold exterior air will ruin my sauna experience.

    I am planning on having an uninsulated wall (but foil sealed?) between the hot room and the change room. I think the air intake will be through a vent in the door, should I be worrying about the outtake vent being in the sauna or will natural convection make sure that I don’t get cold drafts?

  11. Thanks for the quick reply! Just to be clear, it sounds like just a gap under that door will be enough for ventilation purposes in the hot room. If I want to increase the ventilation capacities then I should add the dryer vent?

    The reason that I want to leave the interior wall uninsulated is so that the change room will get warm (but not hot) as well. You suggest insulating this wall anyways and just let the change room warm via the door air gap?

    Rereading your reply also sounds like you suggest also foiling the change room. I was just going to use regular poly in there…

  12. Regular poly in changing room is perfectly fine. Just as with hot room: tape your seams well.
    Insulate interior wall. Changing room will get plenty warm with hot room door opening and closing, or a guy could always prop open hot room door with the water bucket as needed/desired to heat the changing room.
    Add a dryer vent in hot room to encourage air flow from hot room to outdoors without passing through changing room. I’d do it. You may never use it or need it, but if you ever want to, it’ll be there for you.
    Again, there is something magical about meeting super cold dry winter outside air with super hot wet sauna inside air. You may get condensation around your changing room door (to the outside) and this may suck, but with the “bake and breathe” method, you’ll dry out and melt out and win the game in the end.

  13. Thanks for all the great info. I feel much better equipped to build than I did before reading your site and answers. I donated to you yesterday in hopes of getting your ebook but I noticed that it hasn’t arrived yet. It said to bug you if it didn’t arrive soon, so here I am, bugging you for the book.

    Have a great day

    ps. feel free to delete this comment if you wish

  14. Hello,

    This is a great read, thank you for posting!! I am building a sauna in the basement of my northern MN cabin. One of the walls of the sauna will be a pored cement (it’s an exterior facing wall). I am concerned that this poured cement will be damp/create moisture that gets trapped in the insulation and the moisture barrier inside my sauna wall. Isn’t that a valid concern?

  15. Guy: Glad you’re benefiting from saunatimes.

    Yes, a cement wall as one of our basement sauna walls is a valid concern. A cement wall will “pull” heat from your hot room. This is ok if you have a wood burning stove (a bigger hammer) which will overwhelm the conductivity of stone or if you want to heat your walls. But for best sauna performance, for a basement sauna against an adjacent cement wall here’s what I recommend:
    1. Glue and cement screw 2×2 green sleepers vertically against your cement wall, 16″ on center.
    2. Insulate joist cavities with 1 1/2″” rigid insulation (you can easily rip the 4×8 sheet 14 1/2″ wide via table saw).
    3. Foil bubble wrap all your walls.
    4. T&G cedar your hot room walls. The sleepers are your nailers.

    Now, someone may say: “Don’t do that! you want your walls to breathe!” Yea, yea, that makes sense but:
    1. This is done all the time (basement remodels).
    2. Gutters and good drainage outside our homes/cabins keeps moisture from saturating our cement walls, preventing any moisture permeation/infultration to begin with.
    3. Our materials we are using against the cement wall are moisture resistant. No mold if wet. And on that score, after gluing and screwing sleepers against the cement wall, a guy could come back and slop around some Killz with a paint brush. I’d probably sleep better at night with adding in this 25 minute effort to the mix.

  16. A few comments for consideration/with all due respect to home sauna builders, Glenn, and all:
    I began designing and building traditional – style Finnish Sauna here in the U.S.in 1987- and have installed many dozens of indoor Saunas as well, it is a simple thing really.
    And I have had this discussion with builders, architects, and DYI-ers for all of those years – as a consultant, designer, builder, and equipment dealer-and always will return back to Finnish Sauna specs for insulation and vapor inhibitor guidelines: no foam, no plastic, no bubble wrap…period.
    You will have better service using mineral fiber/ or fiber glass, and a foil VI; and the room you create will be that much more authentic.
    Cellar – level hot rooms adjacent to poured walls bring a different aspect, as concrete is both a potential moisture source and also heat sink; regardless, foam of any kind in my view is not a good idea.
    I recommend that DYI-ers contact the outfit where they will buy their heaters/accessories/door, and so on -and ask them how best to prepare and finish the space… if the person on the other end of the phone says’ Wha?, call me-and I’ll help you out.
    I ‘ve fixed many goofed up jobs built by smart well intended folks-and ‘it just ain’t worth guessing’ !

  17. Glenn, my build is coming along nicely. Can’t tell you how excited I am and how much your site has done to help (and inspire) me. Due to the nature of the existing structure I am converting, I would like to line the inside of the hot room and changing room with plywood to add some shear strength to the shed. Having never used foil bubble wrap, should it be installed after the plywood (i.e. plywood, foil, cedar) or before (foil, plywood, cedar). Of course I will have insulation in the stud cavities. Any recommendations? Thanks for everything.

  18. I bought a sauna from sunlighten but it takes forever to heat up. I’m assuming all the heat is going through the roof. Is it safe to insulate the outside of the sauna? My idea was to start with insulation on the top. Any recommendations that don’t include taking it apart?

    Thanks so much for your time.
    Sincerely
    Tina

  19. Tina: tough to know what’s up there. is this some type of free standing indoor kit ? If yes, fire it up and see if you can get on a ladder and feel the top, if it’s just lightly warm, then this, unfortunately, isn’t your problem. If it feels hot, then, yes, you could frame out 2×6 and lay in some R19 and it’ll be a great effort towards not letting the heat escape up and out of your hot room ceiling.

  20. I have made a sauna recently. I went to multiple hardware stores in the area looking for foil vapor barrier. None of them had any and they all told me regular poly barrier would work fine behind the cedar. The more I research the more I am reading that this may not be the best idea. Do I need to redo the barrier or should it be fine?

  21. Grady, I think I responded on another post from you, if you didn’t get it, here’s my response:

    Grady: My cabin sauna (25 years old) is poly. My backyard sauna (20 years old) is foil. I could tell you to take it down or leave it up, depending upon how you’d feel better. In other words, if your at the poly stage, rip it up and start again. if, you’ve already tongue and grooved, move on and you’ll be ok.

  22. Amen, and I hear you. It just started snowing in Minneapolis. I’ve shut down my work laptop. Sauna is at 150f and climbing fast. Two buddies are on their way over. I’ve finished 16 oz of water and am bringing another water bottle out to sauna. Life is good! Glad you’re sauna is completed. Enjoy! send pics.

  23. It is completely finished and working fine. Thank you for your response, it makes me feel a lot better about it. Feels good to have a functional sauna!

  24. I would send some pics but I am unsure of how to do it on this blog. I made the sauna out of an old outhouse that my Great Grandfather made, so it’s over a hundred years old. Turned out great!

  25. Grady: Well done! I’d say converting an old outhouse into a sauna is an example of sauna “upcycling” indeed.

  26. We are just building an outdoor sauna. Had the Brucescrossing stove stored in husband blacksmith shop for a few years.
    We live in western north carolina where the winters are mild, so no wrap no insulation. We are putting a sod roof on it, so that will be the roof insulation. If it does not get hot enough we will insulate wall before sheeting with yellow popular. Can’t wait to test fire it up.

  27. Hey glen I live on the west coast van island right by the water

    I was going to foil barrier then pink fibre glass should I then use a tyvek or tarpaper on wall sheathing?

  28. Mike: Yes. I would foil inside, tyvek or tarpaper outside.

    Now, this is when an especially vocal faction of builders (amateur or pro) will start in with their “let your walls breathe” counterargument (to which I simply don’t prescribe). Building and sealing properly from inside and outside will not allow for condensation or moisture build up or leaking of any kind within the joist cavity, and this is what keeps our buildings safe, dry, and long lasting. How do i know this to be true? I’ve see it with my own eyes. I did some remodeling to our cabin sauna (built 1996) and the insulation and walls looked as new as, well 1996.

    Still, though, I am open minded to counterargument and welcome any chiming in.

  29. but tyvek and tar paper (felt paper) ARE permeable products that allow walls to breathe. that’s why they are used as opposed to, say, plastic sheeting or some other non (low) permeable material. tyvek/felt paper keep the bulk of the water out while allowing any trapped moisture to escape, best of both worlds. unless you are in some arid climate with very little rain, you’ll want a wrap on the exterior. otherwise, get ready for rot/mold. it doesn’t matter what kind of siding is installed, water will find its way through gaps/cracks, wick through other materials, etc.

  30. I am in the process of building my sauna in my basement. I am putting it in a corner of that has 2 framed walls along block walls. There is poly against the block and fiberglass insulation in between the 2×4 studs. I am going to foil wrap my interior and then tongue n groove. Am I ok leaving the current wall situation as is? My thought is yes the fiberglass will be sandwiched between vapor barriors but if sealed properly what moisture would enter to begin with? Also the base plate of this room uses green treated wood. Being on the floor and in the walls do I need to worry about off gassing? Thanks!

  31. I would build the two corner sauna walls independent of the existing walls. Allow ample ventilation between the adjacent walls so moisture is not trapped. Basements are damp by nature because they are below grade. Install a exhaust fan in the sauna that vents outdoors. Don’t cut corners on construction. Do it right the first time so you don’t have to do it again. The fact that you’re even asking the question means you recognize the problem. Build a sauna not a mushroom farm.

  32. Thanks! I put out a few calls and was told this…Since the framing is already set, caulk or spray foam along each stud. Then fill gaps of framing along block w foil backed board insulation. Then vapor barrier the suana like mentioned above. Does this sound okay? I hear about off gassing but not sure that applies when buried into the wall behind a good vapor barrier.

  33. David: You got it. Though i’m no expert, if it could actually get hot enough to off gas behind foil vapor barrier which is behind t&g cedar, well, that would be some serious, serious heat. And beyond all that, with foil vapor barrier well sealed off, you’re really going down a good road. I like it.

  34. Thanks Glenn! I now have confidence in getting this project done and done right! So excited to have my own retreat and get out of the public sauna at the Y! The reasearch has been interesting and really makes this project more enjoyable then just hiring a contractor to do it…n cheaper!! Thanks for sharing your knowledge n experience guys!

  35. i’m not fully grasping the installation you are proposing but be aware that spray foam insulation doesn’t have nearly the maximum exposed temperature ratings as, say, fiberglass. just looked at the ‘johns manville corbond iii’ product data sheet and it is rated up to only 180 degree f. yes, the t&g and foil barrier will provide some heat drop between the hot room and the foam but it would be cutting it close. i would not recommend spray foam in the hot room walls. if you are talking about spray foaming the stud wall against the block and then building a separate wall for the hot room, that would be fine.

    do not place foil barrier on the interior-side of the existing wall with fiberglass insulation and poly on the exterior-side. you will create an ‘insulation sandwich’ and any moisture that gets in there will have a very hard time getting out. it doesn’t matter how well you seal stuff up, moisture finds a way to get in there.

    you mention foil-backed foam board, which i assume is polyiso foam. that has a higher maximum temp, about 300 degrees F so that should be fine for a hot room application but i would still go with good ol’ fiberglass and foil barrier for hot room walls.

  36. Thanks for taking time to express your concerns! Much appreciated!! The spray foam will be minimal at best (just up each stud that is touching block wall). Then I am putting in foil backed insulation board pressed tight to the block wall with foil pointing to interior of sauna. Then I will put up refletrix foil bubble wrap inside the sauna and put t&g over the refletrix. Alot of research and discussion has been done to create decision…something I can feel good about while relaxing in my new sauna when completed. Hope to have it finished before St Patty’s day…🤞🏼

  37. Im living in a condo in thailand and there is a sauna under my condo , the heat goes through the floor and heats my feet are burning if i walk on my floor in the part of condo thats above the sauna , they have no clue hot to insulate , its been making me sick , stupid thai managers dont care , they said they insulated it but it dud not work , can anyone give me sime advice , my email is antcool343@gmail.com

  38. I am currently converting my outdoor shed into a dry sauna and am hoping that I did not make a critical mistake. I used regular Super Six poly vapour barrier instead of foil throughout and now reading this, I feel I have made a major error. T&G is already complete and i do not have the budget to rip it all out. Will this poly be ok? or is it doomed to fail if I leave as is?

  39. Connor: I hope the following allows you to sleep at night (and enjoy every sauna hereafter). We built my cabin sauna, built in 1996, using poly vapor barrier. I am still standing. The sauna is still standing. It smells great, my informal off gassing meter reads “0”.

    A guy can go deep on the interweb and possibly dig up some kind of reported issue with using poly as a vapor barrier within a hot environment. I’m sure somebody has written something about it. However, I simply have had no issue. Many, many, many saunas have been built using poly as vapor barrier. If it were my sauna, i’d not rip it up and start again.

  40. Hi Paul:
    1. no space. I’d fir out your wall with 2×4 and use R13 mineral wool insulation directly against the wall. EXCEPTION: Exterior concrete wall. In this case, i’d use 2″ rigid foam. Why? Exterior cement walls can wick and get moisture and condensation. Need water resistant insulation for this situation.
    2. Wooden panel directly against the foil vapor barrier. Some go crazy and fir out and air gap, but I have never done this nor felt it was necessary. We are paneling with cedar, or a similar ideal moisture happy wood (white spruce). Further, properly vented saunas will dry very well. More on this: https://www.saunatimes.com/sauna-culture/tips/bake-and-breathe-the-best-way-to-keep-our-saunas-germ-free-and-clean/

  41. Hey, I’m a little confused about the thermal insulation. Currently I’m designing a sauna that will utilize a space in one end of my bathroom (pretty long one). The sauna space will contact 3 walls of the bathroom – they are made of concrete air-brick. I have 2 questions:

    1) Does one leave a space between the building wall and the mineral wool insulation layer? Or the insulation layer can directly contact the wall? How to achieve such space if its required?

    2) The insulation layer will be covered with silver foil. Then goes the wooden board paneling. Is it advised to put the paneling directly on the fol layer or there is a need for an air gap between them. Do you need to leave opening slots near floor and ceiling so that air can freely circulate between the paneling and the foiled insulation?

  42. Hi Glen! Im in the process of building sauna in my basement 2 walls be new framed with 2×4 and 2 existing bathroom and garage sheetrock walls. Also will have to frame ceiling to lower it to 7 feet I have 3 questions

    1) What kind of insulation would you recommend to use in the new walls and ceiling?

    2) 2 existing sheetrock walls have no insulations in between, should I leave them as it is or remove interior sheetrock and insulate them as new walls or perhaps put foam board over it and foil vapor barrier over the foam board ?

    3) When installing cedar planks do I need to leave a little space in the corners so when the sauna gets hot and cedar expands(not sure if it does though) making it does not bulges out

    Thank you in advance!

  43. Vladimir: Your question seems very familiar. Maybe someone else asked similar questions recently, but either way.
    1. insulationi: interior walls: R13 batting between 2×4 joists. Exterior walls: 2″ rigid closed cell, cut to fit between 2×2 green firing strips glued and screwed to exterior walls.
    2. Remove sheetrock. Not a friend of sauna (moisture,yuck.. mold). foil bubble wrap over wall studs then T&G.
    3. Butt your cedar tight in the corners. It’s good to acclimate your cedar at room temp for awhile before applying, to minimize the shrinking, etc. If you get separation, you can come back with corner molding, or build with corner molding to hide any seam separation in the corners.

    Hope this helps!. g.

  44. Hello from Duluth–

    We are in the process of converting an old foundation on our property into an outdoor sauna. The foundation is made up of 2′ thick fieldstone, I believe previously used as a pump-house from early days. Currently just four walls that have successfully withstood 120 yrs. Plan is to spend the summer having the masons patch a few cracks, get a slab poured, and a roof overhead, then start the inside work this fall. Many questions floating in our heads for the interior work….

    –The structure has interior dimensions of 14’x14′, pretty generously sized. We plan to build a square (ish) hot room out of one corner, probably in the range of 8’x8′ or 8’x10′ with the remainder being changing room space. Typically, it will be just myself, husband, and the two kids although we’d like to be able to invite friends over….tough to balance desire to keep it intimate/not overally sized, with ensuring it is large enough.
    Any thoughts on the “perfect” size for a typical family use, if space isn’t an issue?

    –We will insulate the hot room as per the usual (ie, build 2×4 framing with insulation and then foil before the cedar). What are your thoughts about insulating the changing room? The stone has tremendous thermal mass, will take awhile to heat up in our northern MN winters, but would also hold the heat well. We plan to sauna a few times per week. If we don’t insulate the changing room, we would likely leave the stone raw which is quite beautiful.

    –The structure itself has no windows or vents, and we’re not about to start cutting holes in this beast. That means that ventilation either needs to be through the changing room or through the roof, leaving out the idea of a vent at eye level to the outside. Do you foresee this as a major issue? Would you recommend trying to vent out of the ceiling/roof, or just through the changing room?

    –We are in the midst of the “feed from the inside or outside” dilemma. I’ve always been one for feeding from the outside, but your website makes an awfully good case for feeding from the inside. My brother in law is Josh of the “flauna,” and he is quite an advocate of feeding from the inside.

    I’m sure we will have many more questions as the project rolls forward (likely slowly, as we are doing most of the work ourselves and the summer will be spent fixing up the foundation and getting electricity and an outdoor hydrant to the structure). I appreciate any thoughts you may have–your website is fantastic! Just purchased the ebook.

  45. Deborah:

    First caution: the 2′ thick fieldstones. Yes, they will hold thermal mass when heated but there are two important matters to consider. 1. 2′ thick stone is going to take a MASSIVE amount of heat to bring up to a temperature such that they will release heat back into the room. 2. Unless the 2′ thick stone walls have insulation on the outside, you will never achieve #1. In cold temps, you can heat your sauna 24/7 and those stone walls will continue to suck up cold from the ground on the outside.

    Please don’t think i’m being a buzz kill, as I love the project and concept. If it were me i would 1) fir out your stone foundation with 2x2s (minimum). 2) apply rigid insulation or foam spray on the inside of these stone walls. 3) t&g panel from here.

    If anyone has a different opinion or insight for Deborah, please start typing. Important we help her do this right!

    PS.. will be on Josh’s pontoon sauna next week! Maybe we talk more on the sauna bench before Burnside plunging. 🙂

  46. Glenn–

    Thanks so much for your reply! We had definitely planned on insulating the stone walls where the hot room is planned, firring it out with 2x4s (actually are considering 2x6s as we have plenty of space and it would allow more insulation)–just didn’t know if we should insulate the stone walls in the changing room area. I think we probably well, as the stone would suck any heat leaked from the hot room very quickly and the changing area would likely feel quite chilly in the wintertime otherwise. My father (the floating sauna builder) was worried about the possibility of condensation given the cold stone mass–I know that normally you avoid spray foam in these settings, but perhaps it would be worth considering in this case given the moisture questions of this particular foundation?

  47. Deborah,
    The stone walls sound like a beautiful thing to work with, but Glenn is right about them sucking up any heat- so you are best to frame in the walls and insulate as you propose. If the masons do too good of a job though they will seal the wall so that the space between the stone wall and your framed wall/ insulation will become a wet, moldy. space. I would avoid fiberglass at all cost – it will be a soggy sponge in no time. Polyiscyanurate (boards or spray foam) is better. No matter how well you seal it, some moisture( vapor) will still get in there. I suggest the masons leave weep holes in the bottom and even perforated drain pipe under the perimeter of the slab. Also, vent holes near the top- sealed with bug mesh. This is basically how brick walls are made- with a lot of holes to let moisture escape either as vapor or water that accumulates at the bottom (it’s also frequently botched in brick walls and there many products designed to help maintain vent space- check a masonry supplier). If you use spray foam, you should still maintain a vented space against the stone as the stone will wick moisture inwards at certain times of the year.
    In the dressing room, I’d do the same but maybe save some stone for a fire wall and have the stove fired from the dressing room so that space gets some heat. The stones around the stove will be just enough to hold heat from the stove. If you look at some of my saunas ( roblichtcustomsaunas.com) you’ll see I use stone facing here- it is very elegant and becomes a focal point.

  48. Glenn,

    My wife and I are contemplating building our own sauna in the basement of our 12 year old home in a room that was originally built as a storm shelter and or gun vault. The room is approximately 8×10 with three walls being below grade comprised of 8 inch cement block that are poured with concrete as part of our home’s foundation. The ceiling is sheet rock with our first story laundry room directly above. My question lies within the cement block walls. I’ve read your comments regarding the 2×2 furring strips and rigid foam. Do you think this is the way we should go? I will be covering the walls with the proper foil barrier and Eastern Red Cedar I have milled myself. It’s going to be difficult to vent other than maybe under an entry door. I’m just concerned about the cement block walls and potential for moisture, although we’ve not had a problem so far. Many thanks in advance! Have enjoyed learning from your site.

  49. Darren:

    Few things here:
    1. Glad you are enjoying saunatimes! All this content is here for you, and if there’s anything debatable, I welcome the chatter.
    2. Sauna in basement. Yes, we need to insulate the cement walls, otherwise you will be heating your sauna until January in order to get good heat. 2×2 firing strips (cedar is best, moisture resistant), rigid foam, foil. Trapping moisture? Well, you haven’t had a problem so far, so that’s good. If moisture does permeate, you are using moisture resistant and mold resistant materials. Good.
    3. Sheet rock in the ceiling: It’s gotta go.
    4. Eastern Red Cedar. Careful here, and please verify. Eastern red cedar, isn’t that what they make gerbil cages and cedar closets from? If yes, (and that’s what I’ve been told) then this material is WAY too pungent for sauna use.
    Read here: https://www.saunatimes.com/building-a-sauna/the-definitive-word-on-cedar-for-our-saunas/

    Let me know how it goes!

  50. Question: outdoor Sauna in Ely, MN non faced insulation about to go up. Was going to use plastic vapor barrier and from what I’ve read. Use foil. Now the next question. Sauna is tyvek wrapped. Non perforated or perforated foil on interior walls?

  51. Mike:

    I like this question. I can get very geeky with the answer, but the simple answer is instead of plastic vapor barrier, I suggest foil. Menards in Virginia, MN sells 4′ wide rolls as well as the critical foil tape for seams. You want to seal that hot room really really tight. No moisture permeating into wall cavities. And you’re going to install venting per the ebook.

    Now, there’s all kinds of talk about walls breathing or vapor barrier – ing. I’m still waiting for someone to argue against me, because I think i’ve got it right. I wrote about it here. And the comments are very good too.

  52. Jon: my thoughts are that you are spot on with your updated plan.

    If I had $10.00 for every sauna build compromised from not having insulated and isolated a stone mass (cement floor, block walls, etc.), I could buy you a brand new Kuuma stove (and it’d last your lifetime).

    We build our saunas one time, and get to enjoy them for the rest of our lives. We want to isolate and produce our own lampomassa. Unless we are building a commercial banya with 10,000 lbs. of stone mass heated everyday, for our home saunas, we can’t afford the BTUs and the time required to try to heat a shit ton of non insulated stone mass.

  53. Glenn,

    I am converting an old fuel generator building at my Northern Michigan cabin into a sauna. It is cinder block for 5 feet on a concrete slab and timber-framed above that. I have installed 2×4 framing on the inside with insulation, but in the corner where the wood stove is planned, I was planning on installing slate tile directly over the cinder block. Upon reading this string, I am thinking I need to install furring strips, foam board insulation, vapor barrier, cement board, and then the slate tile. Thoughts?

  54. I’m going to start this spring. Plan on cement floor and one row of 8 inch block to get it raised above moisture.
    I’ll keep you in questions as I go.

  55. Nick:

    You’re like a Doctor who knows what’s wrong with him, but has to ask anyway.(and that’s ok!).

    You nailed it. Floor is too cold. A concrete floor has a massive amount of lampömassa. Relatively speaking, it’s like you have an iceberg down there and you’re best to isolate it. Don’t stop the ventilation. I’d insulate that floor as you are suggesting. 1″ or 2″ rigid foam. It won’t cost much and you can make it happen. Start with the hot room and see how it rolls.

  56. Hi!
    I have a related condensation query
    Sauna is fully insulated, has barrier membrane, vented internally, freestanding and in an old wooden stable. Stable floor is concrete (with damp proof membrane) but not insulated. There are standard duckboards in sauna and changing area. I have noticed condensation builds up on the inside (warm side) of the full glass door in a band covering 10-50cm from the ground. This happens in winter and summer (outside temp -5 to 25 deg C). The sauna easily heats up to temperature although the lower 50 cm are probably not as warm as I would like them to be. I understand the physics of this: a full glass door is not the best insulation and that the cold changing area (not heated) makes moisture condense on the warm sauna side of the door. My question is: why is there a band of moisture between 10-50cm off the ground which matches the heater vertical range?
    I have increased and decreased the ventilation and the same band appears.
    Could it be because the floor is too cold? Or that I need to stop the ventilation almost completely? Would an insulated liner on the floor help prevent this? If yes, should I line the changing room floor too?
    Any insight greatly welcomed

  57. Glenn – I’m hoping you can help me sleep a little better by answering this question.

    Background:
    I’m located in Calgary, Alberta, Canada. I purchased your e-book, and am working on my backyard, wood fired sauna (8’x12′). As recommended it has a hot and change room. I’ve insulated both the hot and change rooms (ceilings and walls with mineral wool), and foil bubble wrapped and taped the hot room. I managed to find some recycled cedar for the change room, and in my haste didn’t vapour barrier the change room…I just got really excited to see the cedar go up, and likely jumped the gun.

    Question:
    With no vapour barrier in the change room, am I in trouble?

    I’ve got a means to vent the hot / change room after a sauna through the floor drain, and screen portion on the exterior door, but will this be enough to remove the moisture? Really hope I don’t have to take down the T&G cedar in the change room.

    Thanks for your help.

    I love your web-site and pod-cast. It’s been an amazing journey!
    Thanks,
    Jason

  58. Hi Jason:

    If this were my sauna, i’d not take down the cedar t&g in changing room and vapor barrier it. Couple reasons:
    1. you’ve got mineral wool insulation. Much like a Scottish sheep, wool gets wet and isn’t prone to mold. That said, mold can occur if wet and warm for several days.
    2. you are venting your changing room well.

    The best solution is to practice the bake and breathe method for both your hot room and changing room. I use this method for both my saunas, build in 1996 and 2003, and neither have any petri dish behavior.
    https://www.saunatimes.com/building-a-sauna/sauna-tips/bake-and-breathe-the-best-way-to-keep-our-saunas-dry-germ-free-and-clean/

  59. why no insulation or foil? not a fan of this idea. Insulation and foil are not that expensive and provide an envelop of heat and thermal mass containment, which is what we’re trying to achieve with a “hot room” whatever the thickness of our wood (unless it’s 6″ timbers, which is a whole other story.

  60. Located in East Texas, I just framed up what will end up being a 7x7x7 Sauna with Western Red Cedar rough cut 2x4s. I was looking at all the wall options and thinking I will just finish it with 2×6 WRC rough cut lumber and sand it down where I need it smooth. I think I can skip all of the insulation and outside paneling or walls. Any thoughts on this or has anyone tried? If the boards don’t marry up together quite right, can add 1×2 strips to fill in. I’m thinking the thickness negates need for foil, insulation and outside wall. Surprisingly the price comes out comparable. The sauna will be on skids I can move around, but, will usually sit in my roofed, but mostly open barn.
    If you are inclined to reply, don’t hesitate. I’m about to go buy the rest of the wood today. Thanks!

  61. I live in Northern California where it doesnt get below 30 very often at night. I’m wondering about buying an indoor sauna from CostCo (https://www.costco.com/dynamic-bergamo-4-person-indoor-low-emf-far-infrared-sauna.product.100369996.html) and building a 2×6 wall and roof around it except for the door and LED panel. First I would wrap and seal in Foil bubble wrap the entire Kit, then build the faux insulation and weather structure flush against the sealed kit, only leaving the door and Screen panel exposed to elements. From outer-shell of indoor suana, I would start with sealed and taped bubble foil, then R19, then 1/2 inch plywood, then Tyvec, then T&G Siding that matches my house.

    The manufacturers of many indoor saunas recommend a minimum ambient heat of 60 degrees so warming it up is a problem. So, could I just put a space heater in it?

    Am I going about this the wrong way? Should I just be building my own completely from scratch rather than paying $2500 and then another $1,000 in materials plus free labor to get what I need in an outdoor sauna? Or should I just spend the money on an outdoor suana and hope its decently insulated? My biggest concern about building it myself from scratch would be sourcing the right infrared equipment that is low EMF and placing them correctly? Are there templates or howto’s out there? I’d like to build one or buy one that fits 4 people.

    Thanks for your advice.

  62. Hi Ryan:

    I like the creative thinking here. And it’s perfectly reasonable to do what you outline. But I must say, the infrared stuff is not something I’m into. If you’re wanting my advice, I’d be thinking about building your own “shed” as what you outline above is exactly the right material and method of what i’ve done many, many times.

    And when your shed is complete, you can then simply insulate, foil, and tongue and groove, thereby creating your hot room. And with a real sauna stove, now you’re in the game. Miles better heat than infrared light bulbs.

    Everything is detailed in my ebook, Sauna Build Start to Finnish. The fact that you are knowledgeable enough to outline a proposal for boxing in an infrared cabin kit tells me that you could do this.

    It’s a lot more work, but miles better in the long run (my opinion).

    Hope this helps, Ryan.

  63. Robert:

    When building a sauna inside an existing house, in the basement or on a slab foundation, it’s sure easier if we were to just build the sauna atop the existing slab foundation, and no insulation. And we may be just fine with this. We can build a 7’6″ or 8″ high sauna, and have a raised cedar deck floor a few inches above the cement slab foundation so our feet won’t get cold, while sitting on the low bench. So, odds are we’ll be fine.

    However, and this is getting geeky, but concrete is a very poor insulator. It sucks heat. We always want to insulate exterior cement walls, otherwise it’d be like trying to heat a sauna with an iceberg wall. But the floor, it’s a bit of a crap shoot. Warm air rises, and a cooler floor may not affect the performance of heat. Yet, much can depend upon the quality of the electric sauna heater. Not enough KW and we’ll need a spring jacket.

    So how’s that for dodging your question?

  64. can’t tell if those panels are insulated or just solid wood. assuming just solid wood, what you describe should work. i would skip the foil wrap though, not necessary in infrared saunas. you could also just build a roof with a larger footprint than the sauna, sort of like placing it under a shelter. insulation isn’t as large a concern in infrared saunas but this would at least keep the rain off. i doubt that control panel is outdoor rated, might want to tack on a better weather shield locally.

    with infrared, you don’t get very much general ambient space heating like you do with a traditional sauna. so if it is, say, 35 degrees in there, the body parts adjacent to the infrared heaters will get toasty but other body parts will be freezing. a space heater would do the trick and with properly insulated walls/ceilings, it will heat up pretty quick. i would recommend a heater with a fan in it to get a more consistent heat throughout the room. once heated up, you should be able to leave it off when using the sauna. if it is really cold out, you might want to leave it on.

    alternately, you can by infrared ‘parts and pieces’ such that you could build your own structure to your liking and incorporate the infrared components. certainly more work and you would have to have decent electrical knowledge.

    of course, i fully agree with glen and go the traditional sauna route :).

  65. We’re in the planning phase of building a sauna inside an existing house. My question is regarding the existing concrete slab foundation. Is it necessary to insulate under the concrete of the sauna?

  66. Outdoor shed style sauna, electric heater 6X7 interior space, building from scratch. Colorado Front range climate.
    Doing the work myself with oversight from my very experienced home builder friend.
    He’s wondering why not use rigid foam board as insulation for walls and roof.
    Is there a reason to NOT use rigid foam insulation instead of fiberglass, bubble wrap, etc?

  67. Rigid is good in instances where one is firing out an exterior block wall. (2×2’s and then the 1 1/2″ pink rigid).

    Batting is preferred for standard framing joist cavities (R13 for 2×4, R19 for 2×6) as it allows for a better R value for the price. More importantly, I am much more comfy with batting in the ceiling as it is fire retardant. A guy can put a lighter to batting and it won’t ignite. The rigid stuff? I think it starts melting then it can ignite, but i’ve never gone down that road with it.

    In any case, I am a huge proponent for foil, taping the seams very well.

    Your builder friend may get a charge out of reading my ebook. It’s gratifying to have collaborated with many experienced home builders as they come at things from a different perspective, which is appreciated, but often needs a little open mindedness adaptation for when it comes to some of the nuances of sauna building.

  68. rigid foam starts to melt around 200 degrees f, that temp isn’t unheard of in a sauna. yes, the wood interior will help lower the temp such that it won’t be that hot inside the wall cavity at the foam but you could be getting pretty darn close to that temp, off gassing of the foam could be an issue. i wouldn’t mess with it.

    fiberglass melts at temperatures well above 1000 degrees f so no worries there.

  69. bah, replied too fast. per the manufacturer’s instructions, max exposure temp for xps rigid foam is 165 degrees f. i would have to imagine sauna wall cavities could easily get that hot.

  70. i might have not been clear enough-I’m planning on using the polyiso foam not xps.
    Service temps according to mfr spec is up to 250. If my toaster oven gets to 250 I will be shocked.

  71. Hey thanks for posting this useful topic over here, really hope it will be helpful to many to know about the fix on if our sauna walls could talk insulate with vapour barrier or let them breathe. Appreciative content!! This Insultech.co.nz is very useful and its related to what you have actually mentioned here.

  72. right on, mate. Happy to give the shout out to you via your link. Would be good to talk NZ sauna action and how your product performs.

  73. michael, polyiso works but gets spendy. you’ll need 2″ thickness to get r-13 insulation and it will run you $30 or so for a 4×8 sheet or about $1/sf. compare that to fiberglass, one third the cost for the same r-value. sure, need to add some foil/bubble wrap if just going with fiberglass but that is still pretty cheap when added in, half the cost of polyiso. plus the fiberglass is already cut to width with a simple cut to length at home, compared to the board which would have to be carefully cut to stud cavity width and length.

    or would you lay the board over the wall studs? i suppose that could work to but now you are reducing hot room size and won’t have the interior t&g ‘tight’ against the studs. my exterior walls are 2×6 so i get r-21 with fiberglass, would need 3-3.5″ foam board for same insulation level and that stuff is very expensive.

  74. Thanks Matt, there were a few considerations in using Polyiso-most that my toaster wattage was right on the line when considering room size so I was looking for some slight volume reduction.

    I’ve got everything wood run at this point-so the walls are 2×4 studs with fiber batt, then 1/2 in polyiso wall to wall on everything, then another layer of 7/16 osb, foil bubble wrap all taped up and finally my cedar. I’ll check back in when I fire the thing up and let you all know how this worked out.

  75. Hi Glen…thx for all the great info…I’m at the planning/costing out stage of building myself an 8×12 wood fired sauna in spring 2021…I agree with your suggestion to use a warm side vapor barrier, but to be clear, are you meaning the typical #10 poly on the studs first, with the heat reflective foil paper layered on top of the poly, or does the foil paper go on the studs by itself (thus also acting as a vapor barrier, i.e. no poly v/b used at all)? Thanks buddy!

  76. Andrew:

    This off gassing issue with blown in and rigid foam is ongoing.

    The mfrs. rate the product for high temps, and yet there is still a scratching of the head as to its safety in application for sauna.

    For the blown in mobile saunas I am involved with, I sleep at night knowing that my joist cavities are sealed off extremely well with foil vapor barrier. Foil Bubble wrap has an R value and so does cedar paneling. I haven’t put a temp. sensor within the joist cavity to assess where we’re at with internal wall temps, but at some point we have to let go and have faith that what we’re using is not going to kill us. (Tell that to the cigarette salesman!).

    The other school of thought is to go completely free range organic and insulate our wall cavities with Rockwool insulation (harvest sustainably from fair trade sheep), and then we can really sleep well at night.

    But Rockwool insulation should not be used for applications such as basements, where we need to isolate against an exterior block wall, for example. And another example are these mobile units, albeit a horse trailer or ice fish trailer. Blown in creates important rigidity to the structure and does a perfect job of sealing and enveloping. In these instances, rigid is the best viable move, because batting of any kind will trap moisture (basements) or rattle down the road (mobile) and now we’re creating a petri dish of something we really have to worry about (basements) and trailers that will be more prone to fall apart.

    So, there you have it, all obvious chatter, but this is what I know.

  77. Hello Glenn,
    I’m planing on building sauna up in Northwoods Wisconsin this weekend.
    Looks like this kind of insulation has been touched here, but no clear answer.
    I was wondering if I could sandwich 2” xps and 1” Johns Manville foil faced polyiso foam boards in 2×4 walls.? Obviously foil face on the inside. That would give R16 value. Same thing for the floor and ceiling.
    Someone mentioned off-gassing, but I don’t see how foil faced board would Off-gass. Thank you!

  78. Maureen:

    Changing rooms get very moist, so i’d be thinking vapor barrier in this space. Now, regarding vapor barrier on changing room side of the common wall, this is where some may get worked up about “moisture trapping” as you have foil on the other side of this wall, hot room side.

    My take is to poly all the changing room and steam on. Those worked up about moisture trapping need to answer to the question: how would moisture get into the wall cavity anyway? Especially if we tape our seams (foil) and “do da right ting” with the poly.

    Tyvek is made to breathe, and to be able to accept a vapor barrier on the inside.

    Hope this helps,!

  79. Glenn- question on insulation of the change room. New outdoor sauna build, we are just at the stage where the shed is to be delivered. We put a concrete pier foundation , followed by a sandwiched floor of PT plywood, 1.5″ rigid foam insulation, top layer of floor 5/8″ interior plywood. Using an 8 X 12 shed – shell is plywood cladding with exterior board and batten pine, with Tyvek house wrap underneath.
    Sauna hot room will be cedar/foil wrapped/taped /Rock wool R14 insulation in walls, and possibly higher in ceiling.

    Change room interior wall between the hot room and change room will be frame wall, 2 X 4 studs insulated with rock wool R14, foil on the hot room side. Question: no vapour barrier on this wall, correct? as per this discussion?
    Also, the other three walls of the change room: R14 with a vapour barrier or not (poly or paper) for these walls? The interior walls of change room will be clad with TG pine. The whole building as I mentioned will have exterior house Tyvek wrap.
    Thoughts? Would appreciate some advice on this as about to get the shed up next week and then wiring, then insulation so we’re full steam ahead to get things done for the winter sauna season- I’m in Canada north of Barrie, Ontario – lots of snow here starting in December and can get – 20 C fairly often in winter.
    Thanks,
    Maureen
    Thoughts on this?

  80. Hi there, I am planning to convert my cable car in the garden into a sauna. I would like advice please on how to insulate the inside, using up as little space as possible. there is a lot of window. so there is the ceiling and bottom half of 3 sides. it sounds like insulating the floor is less important from what I’ve been reading? the cable car is basically aluminium and perspex windows. i’m in the uk. thanks very much!

  81. Juliet,

    Sounds like a very creative project. The trick with making a sauna out of a shell like this – cable car, horse trailer, shipping container, etc. is how to insulate, seal it off, and apply cedar or other wood paneling available in your home and native land.

    You can get a lot of opinions, but one way to go may be to use machine tapping screws to apply 1×2 firing strips to the existing aluminum frame ribbing, then blown in insulation. Then foil vapor barrier, then wood paneling.

    I could list the things that someone could take issue with, and want to wrestle me social media platforms, or issue me a yellow card, but to help get you thinking, this is how I’d do it.

  82. Nazhip:

    You’ve outlined a real pickle of a situation. Yes, you’ll need proper non combustible and heat shield to comply with setback requirements for this stove.

    You have to forgive me, as I march to the conventional stick frame drum, and in that space, we use durarock, stone board material. This is rigid, of course, and won’t conform to the arc of your barrel sauna.

    I’m wondering about thermal blankets, like what firemen use to protect the heat from your wood stove from the wood, closer than the 42″ clearance setbacks.

    Lame answer, but hoping that’s a help.

  83. Hi Glenn!
    Let me ask you a question about the thermal insulation (above) of the sauna heater. I am assembling a wooden horizontal barrel sauna with an internal volume of about 5.5 m3 (about 6 cubic yards), equipped with a Harvia 8 kW heater. The total height inside the barrel is about 5 feet 10 inches, and the distance from the top of the heater to the nearest point on the wooden wall (which you remember is cylindrical) above the heater is 22 inches. I know right now what will be the reason of the Southern California wildfire the day I turn on my sauna.
    Could you recommend a thermal insulation material for me? The sauna manufacturer does not provide any special thermal insulation. Their instructions simply indicate the minimum distance from the heater to the ceiling at 42 “, which is not feasible in their sauna.
    Best regards.

  84. nazhip, is this a complete sauna kit? if so, the manufacturer should be consulted for clarification. you mention a 8 kw heater so i assume it is electric, these types of heaters are mounted to walls all the time, not sure why it would need to be 22″ minimum away from any wall. similar for the height, every barrel sauna i have seen has the heater located on the end vertical wall so 42″ height clear above should not be an issue either.

  85. Ok, thanks Glenn. All these minimal clearances are Harvia electric heater manufacturers recommendations, barrel sauna producers say they haven’t got any issue having equipped their barrels with this 8kW heater.
    Thanks for your attention.

  86. Glenn,
    I getting ready to insulate my outdoor sauna, standard stick frame construction and my question is about insulating the wall between the hot room and the changing room. I understand the insulation and foil vapor barrier in the exterior walls, assume this method in the changing room exterior walls as well, but is the best plan for the dividing wall?

    Also, are regular paper faced batts ok in the ceiling of both rooms assuming a properly vented truss system?

    Thanks!

  87. Hi Greg:

    Best plan for dividing wall (“common wall”) between hot room and changing room: foil hot room side, batting between joists.

    Paper faced batts: Ideally unfaced batting is better as technically with faced, you’re setting yourself up for a “double vapor barrier” with foil and faced. But all this is gets nuanced fast. I’ve built plenty of saunas with faced insulation, then foil. One comes to mind that is 28 years old and there’s no funny business going on between the wall cavity. How do I know? I opened a wall to do some electrical noise, and it is as nice and dry and happy as the day I built it.

    Ceiling: same. Unfaced is preferred, as you are using vapor barrier. And good on you for the properly vented truss system!

  88. Thanks Glenn,
    I will definitely be using unfaced in the walls with the vapor barrier, was unsure if the foil was needed in the ceiling as it has a bit more opportunity to “breathe” but will do it there too if that’s the best plan. I couldn’t convince myself if I should let the common wall breathe between the two rooms or not also so thanks for the response.

  89. Completely unrelated to this blog post. I have a very old Slevsauna heater unit and can’t find operating instructions for it. The unit came with my 1951 house in Hollywood, Florida. I have always wondered if it safe to put water in it. Do you have any info on this old fully functioning sauna heater? I can’t seem to be able to add photos to this comment box.

  90. Greetings,
    I have enjoyed referencing saunatimes, reading everyone’s work and thoughts and are in the process of building one as I type. We bought the sauna book. One caveat here, we are converting a 1969 two-horse trailer into a sauna. I took care of all the rust and issues with that and now we’re ready to insulate, frame and put windows in. Insulation came up, as it does, and I have read here about Reflectix. With the shell of the trailer obviously metal and wood lined 3/4 down the walls, with a curved front end where the hitch is. With our limited room to move in a metal lined structure, should we insulate just with Reflectix, frame it out with cedar and keep moving, or do we need to make a consideration for the metal lining?
    I’ll post some photos to gain a bit more insight.
    Thanks! It’s been a fun process so far and we haven’t even touched wood!!
    Thanks in advance for the comments.

    G.A
    Clark, Colorado

  91. G.A. – You have a few options here. Depending on what kind of stove you use (and the BTUs getting pumped out – here’s where the Kuuma shines), you might be okay with just Reflectix since it’s such a small space. However, I’d recommend insulation. If you’re comfortable using a foam product, use polyiso (thin sheets in areas where it curves) or high-temp (250 F) rated spray foam. As noted above, find a reputable contractor who knows what they are doing to come spray it. If you want to stay away from foam (as I do), you can also use mineral wool here. Rockwool AFB or Thermafiber SAFB would work. These come in 1, 1.5, 2, 2.5, etc. thickness, so you can find appropriate thickness for your walls. Usually these specific products are special order from construction supply distributors. Or you can get Thermafiber UltraBatt from Home Depot (which comes in 3.5 thickness for standard studs) and cut down the middle to achieve appropriate thickness. Then continue with your foil bubble wrap, T&G cedar, etc….

    Nick

  92. Hi Mr. Glenn, new guy here. I’m familiar with saunas, have used them in the past, but never owned one. I just finished building my last, forever home and am now planning to build a small sauna. I will start with electric since I hope/plan to have solar power someday, but will also incorporate room for a wood sauna stove at some point.

    As I read your blog posts and try and educate myself, I keep coming back to one question that may be specific to me and my location, and that is bugs. Specifically termites and carpenter ants. We have both where I live and I wonder if this is an issue I’m being overly concerned about. While I know cedar is naturally bug repellant, just like pressure treated wood (not planning to use that in a sauna build, just an example) , bugs still find a way to ruin wood. I’m on a slope so to make the base install-less expensive and easier, I hadn’t planned to pour a concrete pad, but build up timber’s, fill it with gravel and then place the sauna there.

    Due to the time and money investment of a quality sauna, I don’t want to be sitting there sweating one day and see any unwelcome visitors. Maybe the extreme temps cure this issue but being new to all this, I’m unsure.

    Thanks for your assistance and have a good day.

    Tim

  93. Hi Tim:

    I hear you on bugs. Cedar, as you point out, is great. And treated is great too, as underlayment. One thought, building on pillars gets you off the ground. Air flow underneath is a good thing. And if bugs are of real concern, you can go around your posts a couple times a year with a bug repellent of choice. In the free range organic space, you can google solutions which include, from memory/experience peppermint oil, tabasco sauce concoction.

  94. Sounds good, Glenn. The option to have it build on posts/piers would also allow me to more easily incorporate a shower platform just outside the sauna door. 😎 thanks for the quick reply.

    One question on your ebook. Does it have a lot of step by step directions? I’m rather handy when following specific instructions, but don’t do too well thinking multiple steps ahead to see the finished project.

  95. The book is divided into 9 chapters. It’s in language for the average bear, not so much a professional tradesman or a total dummy (on either extreme).

    There are some poignant step by step directions. For example, building sauna benches and sauna doors. But there’s room for free flow design and conceptualizing for the sauna builder. This is where/when the magic can happen. (vs. straightforward “grab hammer with right hand, hold nail with left hand” type instructions).

  96. Nick, thanks for doing all of this great research.
    I too have researched all of these materials as I want my saunas to be the best they can be. I’ve also talked to the same folks and downloaded the material data sheets as you have, something everyone should do when working with a new material.

    I have a few observations from years of working on saunas:
    EXP and ESP foam are to be avoided due to their low service temp. I have a melted piece of green foam sitting above my desk, pulled out of a DIY project I was asked to repair.
    Fiberglass may be ok, but if it gets wet, it is a soggy mess and rodents love it. They will chew it, nest in it and pull it out of one place and drag it somewhere else. If you use it 1: make sure your build is rodent proof and 2: make sure you have a complete vapor barrier and are controlling moisture movement. Installing it is also hazardous- those little itchy fibers get everywhere and are not good for your lungs.

    I talked to a couple of the spray foam manufacturers- that too has a lower service temp; both engineers I talked to recommended not using it in a sauna.

    Polyisocyanurate foam board has a higher service temp and the foil face will keep it cooler in its core. I use this material a lot. Just yesterday I had to make some alternations to a sauna I built 8 years ago that has had (high temp) regular use. The foil foam looked as good as new. When I use it I seal all edges with foil tape to prevent off-gassing. It is good for when you have a tight space and the foil face makes it easy to create a continuous foil barrier in your sauna.

    Mineral wool is great and what I am leaning towards now, especially since foil-foam is closing in on $30 a sheet. Rodents don’t seem to like it, it doesn’t produce a cloud of itchy fibers when installing and it is good to 1000° F. You do need to design a wall cavity that will hold it- and sometimes 3″ is hard to come by in tight retrofits.

    Finally, it’s the foil layer—with an air gap— that is important. R-value is calculated on a delta T of normal room temp to outside winter design temp— about 50 °F; in a sauna the temp. difference can reach 200°, skewing that formula. The higher the heat difference, the more important the radiant factor of the foil film. For that to work, the air gap is crucial, otherwise the foil will conduct the heat. As someone who regularly does bronze pours, I can assure you how crucial that thin foil layer on my casting apron is when you are standing 3 feet away from a 2000° pot of molten metal. In a wood burner, if you did nothing else, buy a roll of genuine sauna foil from a sauna supplier (not that polyethylene bubble wrap stuff which melts at 150°!) and install it with an air gap behind your interior wood walls.

    I’ll post some insulation pics on Instagram @saunasbyrob

    Happy New Year everyone!

  97. Hi, everyone!

    What a great site & resource for us aspiring sauna builders. Thank you so much!

    I have insulated my basement sauna space using unfaced fiberglass batting (after making a last-minute decision to NOT use spray foam). I’m getting ready to install the foil-faced vapor barrier. I’m seeing different things online and I haven’t come across the recommendations here.

    1) With regard to the foil vapor barrier, start with the ceiling first? Or walls?

    2) As far as the walls, staple the foil barrier horizontally starting from the floor working up to the ceiling or is it installed vertically?

    3) I did read above to be sure to tape all seams, so thank you!

    4) Furring strips or no furring strips? I see the comment above about the “air gap behind behind your interior wood walls.” I see some people suggest just sort of “bunching” the barrier against the insulation, leaving room to breathe between the cedar and the vapor barrier. I’m seeing more and more that are using furring strips before attaching the cedar. Thoughts?

    THANK YOU & HAPPY NEW YEAR!

  98. Hi Mark:

    1. foil the ceiling first or the wall first, either way. I like to roll the foil around corners, and pushing into the corner with a board, then stapling. It’s difficult to foil tape seams that butt into corners.

    2. Vertical or horizontal work. I do a hybrid approach, generally. Horizontal layer all the way around the walls, then a wrap version vertically up and around the ceiling.

    3. yes.

    4. I’ve built saunas both ways and can’t tell a difference, firing strips or no firing strips (and I’ve tried to tell a difference!). This breathe business seems over emphasized. My cabin sauna, as example, was built in 1996. No furring strips or air gap between vapor barrier and cedar. The sauna walls are as solid and pure and happy as the day we built it.

  99. Thank you, Glenn! You’re incredibly helpful – and knowledgeable! Could you clarify your response in #2 above for me, please.

    So, your hybrid approach would actually create a “double layer” of foil on the walls then? This wouldn’t “violate” the “do not create a double vapor barrier” rule that I’ve seen & heard so much about?

    Thanks again! Your willingness to share your expertise is generous of you!

  100. Hi Mark:

    Re #2, i wasn’t that clear. Only one layer of foil everywhere, but my method has been to run a horizontal layer from the floor, all the way around, putting foil at 4′ up from the walls. Then, I’ll come back and run the foil vertically starting at the 4′ mark, butting against the applied foil (no overlap) up the rest of the ceiling, then bending it to go across the ceiling, and down the other side to meet the foil on the opposite wall.

    The advantage of non foil bubble wrap is that you can overlap it, as it is super thin. The advantage of foil bubble wrap is that you can put it in your cart pushing down the Home Depot aisle. (readily available).

  101. Ahhh, great explanation on the foil method. And then tape the non-overlapping, butted up, horizontal seam (and all other seams). Great idea. Thanks again, Glenn!

  102. How does the use of a sauna foil vapor barrier on the interior face of a sauna hot room avoid the problem of trapping moisture in the wall cavity of a wood framed outdoor sauna that also has Tyvek Home Wrap on the exterior of the plywood sheathing? In theory the foil is a near perfect vapor barrier, and that should answer the question except that a perfect installation of the foil, tape, and other sealants is difficult or impossible. In normal pacific northwest residential wall construction, a single vapor barrier is normally installed on the outside of the sheathing and any moisture that eventually enters the wall is allowed to “dry to the interior.” In hotter climates, where air conditioning is continually used throughout the year , the vapor barrier is installed on the inside of the wall, and moisture is allowed to “dry to the exterior.” In other words, the vapor barrier is placed on the cold side of the wall. With both interior and exterior vapor barriers, any moisture the enters the wall cavity is said to be “trapped” and is likely to generate mold and wood rot.
    Where I live in Seattle, the summers are short, warm, dry, and partly cloudy and the winters are moderately cold, very wet, and mostly cloudy. Over the course of the year, the temperature typically varies from 37°F to 79°F and is rarely below 28°F or above 88°F. The average annual relative humidity is: 76.0% Using Calculator.net, the dew point is as follows:
    % Humidity Dew Point
    10 92°F
    20 115°F
    30 130°F
    40 141°F
    50 150°F
    60 158°F
    70 164°F
    80 170°F
    90 175°F
    100 180°F
    I assume that the hot room temperature during operation will be 180°F and the temperature outside the sauna will range from 37°F to 87°F with a 93°F minimum and a 143°F maximum difference in temperature between inside the hot room and the outside the building. A typical heating cycle would likely be one hour to heat up to 180°F, 30 minutes for a sauna “bath”, and one to two hours to cool down again. The temperature inside the wall cavity will vary over time, rising from the outside ambient by some amount during the time the sauna is heated and then lowering again when not heated. I cannot even guess at how fast the in-wall temperatures will rise and how far trough the Rockwool filled 2 x 6 stud wall cavity the heat will penetrate during a heating cycle. WITHOUT a vapor barrier on the interior face of the hot room walls, I would expect that some condensation would form on the inside face of the plywood sheathing year around because the wood temperature would always be below the dew point, even if the humidity inside were as low as 10%. With a well sealed foil vapor barrier on the interior face of the hot room walls, I would expect some initial condensation on the foil until its temperature rose above the dew point when that condensate would begin to evaporate again. So, I think there is a strong case for installing the interior foil vapor barrier, especially if the finished interior wood paneling is attached to batten boards over the foil with the nail holes attaching the batons sealed between the face of the foil and the head of the nails (and the nails attaching the wood paneling penetrating only into the battens).
    This leaves the question of what to do on the exterior. Right now I have Tyvek over the 1/2″ CDX plywood but no finish siding yet. Do I leave the Tyvek on and finish install the siding? Do I take the Tyvek off and replace it with a more permeable underlayment? What do you think?

  103. I need to make a correction to my prior post. “In cold climates and during heating periods, building assemblies need to be protected from getting wet from the interior. As such, vapor retarders and air barriers are installed towards the interior warm surfaces” “In cold climates, air barriers and vapor retarders are installed on the interior of building assemblies, and building assemblies are allowed to dry to towards the exterior.” “In hot climates and during cooling periods the opposite is true.” Builder’s Guide to Mixed Humid Climates, p. 111, Joseph Lstiburek, Ph.D., P. Eng. 2005.

    After brief research, I conclude that I will keep the Tyvek because it has a perm rating of 58, and should allow any moisture that poses through the semi-permeable plywood to dry to the exterior. See, https://inspectapedia.com/interiors/Perm-Ratings-for-Materials.php

  104. Greetings,
    Thanks for all the info on this post. Great to read. I have a question regarding the Rockwool. I’ve read the Rockwool Comfortbatt is good for saunas and you had mentioned the Rockwool AFB. One is an exterior and one interior insulation. With our horse trailer sauna having metal walls halfway up the sides and full metal ceiling which would suffice? I guess I’m having a hard time deciding which to use as the trailer is so different than a framed structure.
    Thanks crew!

    G.A

  105. The Comfortbatt and AFB are basically the same product. The AFB is slightly more dense, but they have the same R-Value. Since AFB is marketed as an interior product, Rockwool doesn’t list an R-Value for it. However, if you reach out to their customer support (which I did), they confirmed the R-Value is the same as the Comfortbatt. I used AFB because of the minimal depth I needed for my walls on my trailer (2 inches) and I was able to find it at a construction distribution warehouse. However, Comfortbatt would work fine. You would just need to cut them down the middle for the depth that you’d need (which would be somewhat of a hassle, but guessing you don’t need that much for your horse trailer).

    Hope this helps!

    Nick

  106. Nick!
    Thank you so much for this information.
    Are you building in a trailer too?
    Ours is a 1969 two horse trailer, so plenty of rust and patch work needed, but that’s out the way and now to the insulation and framing! Pretty stoked about it!
    I’ll be a regular with questions on here, so thanks again!
    Till next time.

    G.A
    Clark, Colorado

  107. Glen,
    Thank you so much for your website! I have been reading through all the comments in effort to not ask the same question twice and I must admit that I think my question may have been answered…

    I am building a 5x5x6 foot sauna or rather my son is for his high school senior project.

    Our original plan: Inside walls will be 2×4’s with wool insulation, foil barrier taped at all holes and seems, furring strips to allow an air gap for condensation and then tongue and groove cedar. We were hoping for the outside wall, on the other side of the insulation to just be that hardwood plywood that we are nailing to the other side of the 2×4’s. . Is having the outdoor hardwood plywood serve also as the siding a bad idea? I think you are going to suggest that we Tyvek it and find an affordable siding to go over the Tyvek? Also, do you recommend furring strips again between the Tyvek and the siding? I heard that mentioned above.
    Wanting to keep it simple!
    Also, just the door cracked open after the sauna has been used, could that provide enough ventilation after use?

  108. Hi Krista:

    Wow, you are well researched with your project!. Well done.

    Good plan on the inside, you’re in good shape there.

    Plywood exterior sheathing is a great idea. CDX is fine if you’ll be siding over the sheathing. And yes, i’d Tyvek. And I personally think the firing strips between Tyvek and exterior siding is overkill. Tradesmen may scold me for this, but I like a tight fit out there for de-critter-izing.

    I like your spirit of keeping it simple.

    Ventilation: I would definitely install vents. I like eye level while standing, ear level while sitting on the upper bench, with chutes like this.

  109. Hello,
    Just wondering if it’s necessary to sheet the exterior walls for an outdoor sauna? Is it enough with just tyvek paper and cedar siding?

  110. Tyvek is a breathable material, made just for this purpose. I like using it over exterior sheathing (plywood) and under siding (cedar lap or otherwise). We only have one opportunity to apply this material, and the square footage of our saunas are not large, so I think it’s worth the small investment.

  111. Hello,

    Thanks for this great information. I am building a sauna in my shed and am waiting for the aluminum foil radiant barrier to come in the mail. However, with my research, I came upon this video. It looks like the builder did a plastic barrier first on the insulation and then the aluminum barrier ontop. Is this recommended at all or should it be avoided?

    https://www.youtube.com/watch?v=yTUxZoPJdQY

    Thanks in advance!

  112. Hi Marc:

    A “no no” in the world of building is double vapor barrier. Doing this has the potential for trapping moisture.

  113. Help! We didn’t have time to start insulating before the rain came and the entire inside of our outdoor sauna is covered in either mold or mildew. What do we do?

  114. Hello to all
    Amazing what can be found on the Internet while stuck inside the house on extremely cold , snowy days.
    Have just started thinking about a sauna build. Have tried the hot tub route , (something always needed to be repaired, maybe we got a lemon) Anyway have a good friend extremely good at carpentry,( he does wood I do steel and metal what a pair ) and thinking of a 6 ft x 8 ft sauna with electric heater. It is not normally this cold here in Ks , will be down around 2 below this weekend From all of the above comments I assume 2×4 framed wall is sufficient. I plan on a steel frame work with wood being attached to that and the steel base anchored to the patio concrete. Any thoughts and or suggestions welcomed.
    If anyone would be so kind as to forward any addresses, and or e mails with companies names that supply heaters and accessories I would be indebted
    Thank you all so much

  115. Hi there. New to the forum. I am doing extensive reach on sauna insulation and came across your site.

    Due to my “build” being a shipping container. I am aware closed cell foam spray works extremely well against the steel and achieves a zero air gap. Having gone through technical data from leading manufacturers. The foam achieves a heat deterrent upto 90 degrees.

    Now my thoughts for the wall build up. Is to batten the internal face of the steel, spray foam, heat radiant barrier. Then instead of timber rips to attach the cedar. I’m looking at a cement board so I can do a herringbone style pattern. The cement board has extremely high heat and moisture resistance.

    Please let me know your thoughts.

  116. Hi Glenn, I bought your ebook several years ago and I’m finally seeing the light at the end of the tunnel (it’s faint). I’m converting an 8’x12′ shed into my sauna and have some questions about insulation, vapor barriers, etc. The shed has untreated T1-11 fir siding with no Tyvek or house wrap between the siding and studs. I’m planning on insulating the stud cavities with fiberglass bats, then foil bubble wrap in the hot room (seams taped obviously), finished with cedar T&G. My questions are:
    1) Will not having a form of house wrap between the exterior siding and studs be problematic? I live on the eastern side of the Cascades in Washington. We get a fair amount of snow in the winter, but not much rain the rest of the year if that makes a difference.
    2) Should the fiberglass insulation be faced or unfaced? Any difference between hot room exterior walls, ceilings, hot room/changing room common wall, and changing room exterior walls?
    3) Should I have a vapor barrier between the interior siding in the changing room and the studs/insulation? If so, any specific kind?

    I’m an ER nurse and I’ve been working in California for the past 6 weeks, but I fly home on Sunday and plan to work into the night every day until I’m done. I’ve got a garage full of materials and my Kuuma stove should be arriving next week. The one thing I don’t have yet is the fiberglass insulation, so wanted to get this sorted out so I can hit the ground running. Thanks for all your insight.

  117. Hi Jacob:

    Good on you for the intention on making it happen!
    1. No worries on no house wrap.
    2. Unfaced.
    3. Changing room vapor barrier: can be standard poly.

    You can make many other mistakes, yet given you have a Kuuma arriving next week, well, the Kuuma cures all ills. So happy for you. Bringing great lämpömassa to the Cascades. Can I come visit? Post-corona-good-heat-sauna-tour is in my future.

    Happy you’re hitting the ground running, Jacob!

  118. Appreciate you getting into the weeds. I have consulted on a few shipping container projects. The concept you detail above is close to what I’m feeling good about. heat radiant barrier (foil) is a good insurance policy against any yuckiness that may happen behind (and it’s a stretch as to what, if any, yuckiness can happen as the service temp, as you point out, is within range for spray foam.

    But what i’d be doing is cement board for the stove surround and that’s it. Too much cement board is too much heat conductivity. We want to balance that. Extreme illustration: A sauna in a cave doesn’t work, all the heat goes into the stone and earth. But the right amount of cement board, thermally isolated and around your sauna stove, creates an awesome degree of lämpömassa that you’re going to feel in your bones and I guarantee that because you’re going to build your shipping container the right way, when you hit your sauna rocks with a blast of water, you won’t be able to say anything but “aaaaaaahhhh”- at least until the steam passes (and this is a good thing). And this is one of the key characteristics that separates good sauna from lame sauna.

  119. Glenn, you’d be welcome anytime! The least I can do for all the imparted wisdom.

    I’ve been shopping around, and for whatever reason I can’t find unfaced insulation around here, my local hardware store only carries it in R-19, and the big box stores only sell it in bulk volume for an ungodly amount of money. What’s the downside of using faced insulation, and does it change what you do for the vapor barrier?

    Thanks

  120. Hi Jacob:

    I’ve been in a similar pickle and have procured and used faced insulation for several builds, then foil. These saunas are all still rocking. (I was just in one of them last week!). Because you are taping the seams of your foil, and sealing off any moisture permeation, you will be eliminating any moisture trapping between your foil and kraft paper.

    Using faced insulation doesn’t change the vapor barrier set up. Faced insulation is easier to work with. It fills the joist cavities well. You’ll like it. Keep it going, Jacob!

  121. For an outdoor sauna should you use vapor barrier inside and outside? Traditional homes have a vapor barrier between the exterior siding and frame sheeting and then a second plastic vapor barrier between the interior frame and drywall. Would using both vapor barriers keep out all moisture from inside the wall or would it trap moisture in the wall?

  122. Hi Chris. No double vapor barrier. Foil the inside, and Tyvec or some breathable membrane or nothing on the outside.

    Correct, If you were to apply vapor barrier to both inside and outside, it would trap any moisture in the wall and that’s a big no no, as you can imagine.

  123. Thanks for the response Glen. What about ceiling insulation? Is traditional fiberglass insulation recommended or would a spray foam insulation be better? My initial plan is to build a cathedral ceiling with soffits and baffles in the changing area and a traditional ceiling in the the sauna. I’m wondering if a spray foam insulation might better seal and protect against drawing up moisture from the sauna instead of the fiberglass.

  124. Hi Glenn (and all Sauna Times contributors),

    Thanks for your e-book and passion in managing Sauna Times.
    I’m planning a conversion of the bathroom in our “up-north” garbin to a sauna.  The structure is 2×6 construction (on a slab), fiberglass insulated walls, 1/2″ plywood sheathing, tyvek exterior wrap, and vinyl siding.  The interior has a poly vapor barrier and latex painted sheetrock. 

    My question has to do with how much demo’ing and reconstruction is needed/desired for moisture management.  It seems that the most labor intensive approach but safest would be to tear out the sheetrock and poly vapor barrier and replace with a foil vapor barrier and suitable hot room wall covering.  The question is, do I need to remove the sheetrock?  Instead, would either of two following options be viable:

    Add a foil vapor barrier over the sheet rock and cover that with a suitable hot room wall covering.  Or might this cause problems with the foil and poly separated only by sheeetrock?
    Add suitable hotroom wall covering over the sheet rock (no additional vapor barrier).

  125. Hi Leroy:

    Others may chime in, but my suggestion is:
    1. Remove the sheetrock & poly.
    2. Foil it. Tape the seams.
    3. Leave the gun. Bring the cannolis.

  126. Hi there, very interesting reading.

    I am going to start building an outdoor sauna and plan on using bubblefoil inside the steam room, walls and ceilings.
    T&G cedar on top of foil.
    My question is: Could i use foil only and no insulation on exterior walls?
    Don’t know yet what exterior siding I’m going to install.

  127. You can foil, then air gap or no air gap, then paneling. Then get after your joist cavities from the outside, when you figure out what exterior you’ll be going with. This is unconventional but possible. Hope this answers your question, as I think i got what you’re thinking.

  128. Thank you Glenn, yes you got it. it’ll be some months before i get it completed but will post here to let you know if it was a good idea or a fail.
    I have not decided what to use for exterior siding yet.

  129. Mark,

    I hear you. Finns like an air gap under the hot room door big enough for a cat to crawl under.

    I’m more into a gap under the hot room door big enough for a rabbit, like maybe 1-2″.

  130. Hi, Glenn:

    Would you kindly help me make sure I get something right the first time, please! I am seeing some doing air intake gaps under the sauna door as small as 1/2” wide and I see more so lately that Finn’s (my people!) are now doing a 4” gap which seems to me to be a lot!

    My sauna that I am having built in my basement is 7.5’ L x 4.5’ W x 7’ tall. I have a 8KW electric heater (yeah, a bit big for the size I’m sure, but I wanted the Bluetooth capability.)..

    The heater will be right next to the glass door that I’m having made, so the air intake will come from under the door rather than under the heater.

    Could you please give me your best advice as to what the airspace gap should be under the 24” wide glass door and then what size outlet vent you would recommend on the back wall under the top bench? I don’t want to screw up the door gap as it’s expensive and I need to make it right the first time.

    Thank you!

    Mark

  131. Haha! That’s awesome! Thank you, Glenn! What size would you recommend for the outlet vent? I see some are more square at 5” or so while others seem to suggest the more rectangular wall or floor register size.

    You are always so tremendously helpful. Thank you so much!

    Mark

  132. Thank you, Glenn! Could you help advise what size you might recommend for the outlet vent? Thanks so much, you’re always so tremendously helpful!

  133. Hi Mark,

    My experience is that it can be easy to get very nerdy/technical regarding what size vents and openings. Yet this article installing vents is as easy as 1 2 3 details what I believe to be a really good vent system. One down low, and two up high seem really good. And the ability to tune the air flow with the chutes is a great way to go.

  134. Hi Glenn, thank you for your website.

    Before I follow all the rules I need to understand one more thing. The climate I live in sounds almost polar opposite to yours and many people building saunas online.
    I am in Perth, Australia.
    Google explains it best “Perth has a mixture of the Californian and Mediterranean climates, with mild , rainy winters and hot dry summers. Perth is the sunniest capital in Australia with an average of eight hours sunshine a day.”
    In winter, the coldest month our average temp range is 10 – 19°C (50 – 66.2°F).
    Damp and mold issues are not a common issue here as they are in colder or humid climates.
    So, my question is, does this impact advice on sauna build and the threat of condensation? I’m wondering if the insulation, foiling etc is giving a small increase in heat retention, for added complexity. Are there certain climates where letting it breathe makes sense or no way?
    Thanks again, learning a lot!

  135. Hann..

    You’re onto something. Letting walls breathe is a contentious issue with sauna builders. In Minnesota, we are building saunas and stoves like armory and rugged tanks, ready for blizzard battle, and the quality of the heat shines through. Your climate reminds me of California. And in California there are lots of wimpy saunas, barrel saunas, toaster ovens. As much as I shudder, these “saunas” don’t show their deficiencies much due to the temperate outdoor climate.

    So, yes, Hann, you have a lot of runway with your building options out there, and “good on you” for advancing with sauna!

  136. Hi, I’ve posted a comment / question a few days ago and on your contact page it says the best way to contact you is through one of these blogs. I know this is a free service, but I would love if you could answer my questions about an air barrier between the vapor barrier and interior planks and also about my sliding glass door application. I’ve held off working on the spa hoping to hear your opinion before I go any further. Thanks in advance!

  137. Hi Glenn,
    I have a question on polyurethane – the wood finish- not the insulation (sorry wasn’t sure where to ask this question).

    I have just bought second hand cedar panels that are beautiful and in great condition. The seller assured me they were raw, and only after purchasing did I realise that one side of each lining board is shiny.. I’m guessing it has had a polyurethane finish applied to one side.
    This was confirmed when I wiped the boards with a damp cloth. Water soaks in easily on the raw side, but not on the finished side.

    I have read this is a no no in sauna. And though the boards are over 5 years old and probably properly cured, I am worried about having any chemical inside the sauna, after reading about ‘offgassing’, at high temperatures.

    I can’t return the cedar, so my options are:
    1) Install the lining boards so the raw side faces inside of the sauna. Boards are 10mm thick, V-joint – maybe this is an ok seal?
    2) Laborious sanding of each panel, then again install raw side faced internally.
    3) It’s not salvageable, buy the correct material.

    Thank you, your book and website has been very helpful so far!

  138. That’s weird my first comment said it was in for approval and hasn’t shown up, but my second comment showed up right away. So anyway, I will rewrite my first question lol:

    I have read that there should be an air space of some sort between the poly reflective vapor barrier and the interior T&G. Is this true and if so, how much space should it be? Should I just staple the barrier in between the studs? This seems to negate the taping and full vapor barrier idea. Or, should I add furring strips on top of the VB? Or can I just forget I read that and apply the interior boards right over the VB?

    Second question:
    The sauna I’m building is an extension off of a covered deck / 8′ wide sliding glass door. I already have the room framed out and I’m getting ready to sheath and insulate. The room is approximately 8x8x7h. The existing slider is not new but not super old. It’s fairly tight (I’ve been in homes w lower quality ones before with air gaps, etc) and insulated glass panes. I really wanted to use the doors as the access to the sauna and for the light aspect. Will this be ok or do you think it might lose too much heat? I seem to be at the top end of the scale of size on most sauna heater specs, so I’m also wondering if I should up say from the 9kw to the 12kw, where the room size is at the lower end of the scale. One rep has been telling me the 9kw will work, but I think he’s trying to unload a floor model on me. I’m also still waffling between electric and wood heat. I’m just concerned with wood that I won’t be able to regulate the temperature as well and also thinking about the ventilation and cleanout aspects. We have a small wood stove in our shed and a large one in our house so I’m fully versed in wood stove maintenance.
    Thanks in advance for your help and advice!

  139. Esko, so, if you put your hand against the basement wall, against the ICF, if you’re hand is touching poured concrete, well, instead of plywood, i’d be thinking of glue and screw 2×2’s 16″ oc against this wall, then 1 1/2″ rigid foam ripped in table saw between the joist cavities, then foil, then t&g.

    Cement is a heat suck, and it’ll be important to create an insulative barrier, retaining all the heat within your hot room.

  140. Hello, I have a question on building my sauna in my basement. I live in Sudbury, Ontario. My basement foundation is made of ICF (insulated concrete forms) where the forms are insulation (about 2.5”) on the outsides with 6” of poured concrete along with rebar on the inside. It is insulation, block wall, and vapour barrier all rolled into one. I was thinking I would just use plywood against the ICF and then foil and then my cedar. Would there be any concerns finishing it this way?

  141. I left a comment / questions twice now on this forum and have been waiting for an answer so I can move forward with my sauna build. I’ve checked every day and no response so I thought maybe you were unavailable, but today when I check another person has posted a question and it’s been answered right away. Have I done something wrong? Please advise. thanks

  142. Hi Glenn, thanks for the quick response. I was just late in checking back. The ICF has insulation on both sides (inside and outside) so I have about 2.5 inches before I hit concrete. The ICF has screw points every 8”. There is an insert that accepts screws that I can attach the plywood to. Drywall is installed right now due to building regulations to cover the styrofoam. I will remove drywall, install plywood and then foil and cedar. I was a little more worried about 2 vapour barriers back to back. The ICF and the foil. Would this be a problem. Thanks for your help.

  143. Amy:

    Vapor barrier:

    This is detailed in Sauna Build, Start to Finnish, you’ll want to go in sequence:
    1. insulation.
    2. foil vapor barrier (no poly in hot room).
    3. spacers.
    4. t&g paneling.

    #3 is amongst debate as to whether necessary or not. One nuanced argument for spacers is that they are better to nail into when paneling as fewer penetrations into foil. Another is reflective vapor barrier works better if an air gap. Again nuanced.

    Sliding door:
    I have never seen this in sauna, so, I can’t say that it would work. My instinct is to nix the slider, and install a standard out swing hot room door. Plans on how to build a decent sauna door are detailed in the book.

    Hope this helps.

  144. I wish I could understand why my comments aren’t being responded to at all, but everyone else’s seem to be responded to almost right away.

  145. Hi Esko:

    Yes, rip out the drywall. Yes, foil against the foam, then strapping and cedar. For sure. I like all that. Sauna on!

  146. Hi Glenn, I have been thinking. Sorry. Can I just rip out my drywall off the ICF and install foil against the 2.5” of styrofoam which is part of the ICF and then strapping and cedar instead of installing plywood? It would save me a few bucks if I could do it that way. Thanks to your forum. Very informative.

  147. Glenn
    Thank you so much for your help on my questions! I’m going to go ahead and try leaving the sliding door and I will let you know, if you’re interested, how it works out. I can always change it out after the build without having to redo or rip anything out that’s already been done, so I don’t have much to lose in trying it.
    Thanks again!
    Amy

  148. Hi! We are in Northern Ontario and our unfinished outdoor sauna is concrete block exterior walls. We have concluded that applying vapour barrier to the wall studs, then insulation in between studs & foil barrier after that before cedar T&G, is the best scenario. What do you think? It will get used in the winter and are concerned with the concrete walls sweating in winter causing moisture behind the walls. Do not want to insulate on the outside, prefer to have cedar finishes inside.

  149. you’ve got some good thinking here.

    block walls will sweat, most likely.

    It’s important for a thermal break between these walls and your hot room.

    I would screw in 1×2’s into the block 16″ oc, and fill the cavities with 1 1/2″ rigid.

    Then foil.

    Then t&g.

    Nobody has convinced me of a better way to deal with stone/cement walls and sauna. I’m still open for other thinking, though.

  150. Hi Glenn

    I’ll write on this thread as the article is more related to my current consideration.
    We had been discussed insulation for a stone / brick structure here in India. You had advised against using mineral wool / rockwool and suggested rigid foam. Is that the foam that is discussed in this article?

    Reading these various options, none of them seem overly suitable and risk free in terms of gas and other toxic aspects. Is the type of rigid that you suggested different to the types considered here in this article?

    Here in India we have 3 months of extreme rain during the monsoon and 9 months of baking heat.
    The period of the monsoon is of the most concern for my stone outhouse and the most suitable insulation for this. The walls are made of 5″ laterite stone.

    Thanks for this great resource.

  151. Hi there

    I have been considering the correct insulation for my stone / Brick walled sauna.
    It had been suggested to use rigid insulation and not Rockwool with stone/brick walls.
    After reading this article it does not seem any of the rigid foam insulation options discussed seem particularly non-toxic and safe. The Rockwool seems to be recommended.

    Is there another type of rigid insulation not discussed in this article?

    Also, have you ever heard of somebody in a hot country not using insulation and just making a frame and fitting aluminium bubble foil or other similar aluminium vapour barrier and then the cedar over that?
    How much heat passes through the aluminium bubble foil that would be absorbed by the brick walls rather than being stopped by an additional layer of insulation?

    Thanks!

  152. Hi Danny:

    I like the idea of screwing in firing strips into your brick wall, 16″ oc. Then 1 1/2″ rigid insulation between the firing strips, then foil, then wood cladding paneling. And if you like, before paneling, another course of firing strips to create an air gap between foil and cladding/paneling.

    Now all this may seem crazy in a hot country, but good on you for getting into sauna. Is it like enjoying spicy hot food in hot climates?

    I’d be thinking ice baths between rounds, right?

  153. Hi there!

    This is not at all crazy. We are regular Ice bath users ( which we made from a converted chest freezer ) so the Sauna is the perfect addition to that. Also one of our team is a Swede and we spent a lot of time in Sweden, Finland and Estonia and the benefits of a sauna are way beyond the outside climate. Although its hot here for a lot of the year, the monsoon time is very damp and in the winter the nights are chilly ( compared to summer here anyway! )

    The plan sounds great but I am not 100% sure what the rigid insulation is?
    Is rigid insulation the various plastic foam options that are discussed in this article or is there something else?
    Are speaking about polyurethane, polystyrene, or polyisocyanurate foam?

    Any advice on specifically what to search for here in India as a good rigid insulation would be awesome!

    Much thanks!

  154. Hi Glenn,

    I’ve been reading up most of your articles in the last day or two. I happened on your site after researching hyperthermic therapy. It’s being studied for treating a wide range of health issues from cancer, back pain, viral infections (link to neurodegenative diseases such as Alzhiemers, ALS, etc) hypertension, and athletic recovery. Here in New Mexico, a native American sweat lodge is the closest thing. But looking for something more efficient, I came across saunas.

    I saw on Youtube that in some builds in Europe, they use an foil covered foam board called Sauna-Satu. It is heat stable at sauna temperatures, up to 100C, and is made of polyurethane. Off gassing/emissions rating is M1, which I assume is the lowest category. The problem is that it’s expensive, at ~$20 plus shipping from Europe, for 42″x24″ panel. That’s about 8 times more than the pink boards above.
    https://finnmarksauna.com/products/kingspan-sauna-satu

    So I was wondering why not use hardiback boards, which you use around the stove and the floor, for the whole thing. It’s cheap, has more thermal mass which, once heated, imitates a log cabin for that deep feeling heat (Infrared radiation). And you don’t have to worry about vapor barrier. It’ll look like a bunker, but you could add floating wood panelings. If you attach the wood panelings from two adjacent walls, then they would be free standing.

    BTW, thanks for this website. It’s been informative and motivational!

  155. Hi Carol:

    Love your creative thinking re: hardiback board not just for around the stove but for the entire hot room.

    Now, let’s take the aesthetics off to the side, well, not quite yet, let’s talk aesthetics first. Yes, it’ll look like a bunker, which we’ve addressed above, and yes you could face with wood. Alternatively, we can detail hardiback/cement board/Durarock (words of same thing) in some very cool ways. We can face it with tile or cultured stone, or skim coat it with cement (a personal favorite), and it can arguably create a nice looking space. so, there’s that.

    As to the core of your consideration, you are spot on. This material is inexpensive and has more thermal mass, for sure. It can act as a vapor barrier, yes (assuming we seal off the seams).

    Now, Carol, i’m racking my brain on this one. There is time to heat this mass. And as long as this added mass is isolated (ie thermal break), I suppose it wouldn’t take that long to heat. And you are aware of this:

    “Metals and stone are considered good conductors since they can speedily transfer heat, whereas materials like wood, paper, air, and cloth are poor conductors of heat. … Materials that are poor conductors of heat are called insulators.”

    So, if you have two saunas, all the same except A) Sauna is hardiback/cement board/Durarock with insulated wall behind, and B) sauna is Kinspan with insulated wall behind, well, The A) sauna will take longer to get up to “serving temp.” but i’m not sure it’d take that much longer, as we have a thermal break working in our favor, and once this mass is hot, well, as you mention, we have good lämpömassa and lots of “ahhhh” for when we toss water on the rocks. (vs. toaster oven lame heat).

    And given your attention to and concern for using materials that don’t off gas and have low emission ratings, I’m thinking that you’ve researched that hardiback/cement board/Durarock doesn’t have yucky, jankiness material in it, so we’re ok there..

    hmmm… you’ve got this grey haired sauna builder enthusiast kind of stumped.

    I have recommended hardiback/cement board/Durarock beyond the stove surround as knee walls in the hot room before, but more as a cost savings concept than for thermal superiority.

    Carol, you got me here. Now the rest of my day is going to be on this subject. I’ve got a thermal engineer on my team, and I have added this concept to my list for him. He’s as crazy as I am.

    PS.. I am emailing you separately re: hyperthermic therapy. http://www.saunatudies.com.

  156. Glenn,

    Thanks for your thoughtful response! You totally grasp what I’m thinking. Crazy is right. Some folks don’t like out of the box ideas. But you’re open to them. I appreciate that. I’m looking forward to where this leads…

    One thought I’m discussing with my husband is whether installing the heavy cement board on the ceiling would be a good idea, or not. I’ve only worked with cement board once, a small tiled piece for an indoor water fountain.

    P.S. Also thanks for the link. Looks like good info. Looking forward to your email.

  157. Glenn

    I wonder if there is any consensus between using rigid foam and adding a separate foil vapor barrier when compared to using foil backed foam.

    the Finn Mark foam for example seems super easy to use and they seem to be screwing it directly into the cement wall and then adding framing over the top for the purpose of adding the cedar.

    Any insights?

  158. Thanks to a lot of information gathered on your blog, I will shortly start my outdoor sauna construction;

    Outdoor, 6×8, no changing room, 6K heater (wood fired not allowed in our town).
    Canadian climate (-20F to 95F)

    Floor construction from bottom to top:
    6in gravel – deck blocks – adjustable supports – treated 2×6 floor & 2in rigid foam (or more) – 3/4 treated plywood – 1/2 cement board + Quickrete vinyl cement.

    Wall construction (inside to outside)
    Cedar – Vapor foil – 2×4 walls (treated bottom plate) & insulation batts – 1/2 OSB – house wrap – Cedar

    My main concern is the roof, as I’m building a very low slope metal roof and not fully confident as it is not discussed often on the blog; I’m essentially thinking of constructing it as a thicker “wall”
    So from inside to outside:
    Cedar – vapor foil – 2×6 roof structure completely filled with batts insulation (unvented) – 1/2 plywood – “Deck Armor” underlayment (roofing breathable membrane) – Corrugated steel roof.

    Essentially wondering if condensation management will be efficient ?

    Let me know if you see any major flaws in my design and/or if you have any recommendations.

    Thanks again for this great blog.

  159. Glad you are enjoying saunatimes.

    Building a roof thicker for more insulation is a good line of thinking.

    More insulation up top, to contain heat rising – good.

    No ventilation – Yes, that is something. The heat differential may want to create condensation. We get that. But because sauna is a space that is heated only intermittently, a handful of hours a week or so, all the material will be most of the time at a “steady state.” And given that you will be venting your hot room well, and completely vapor barrier’ing your hot room, I’d sleep well at night knowing that the insulation in the ceiling rafter cavity is doing fine.

    I don’t think the potential benefit of an air gap between steel and roof plywood would be worth it, but that would be my only thought for you.

    Now, the only thing that may suck is that an electric sauna heater, in winter, may have to fight like the “little engine that could” to get your hot room up to temp. It’s not so much about the ratio of Kw to 6×8 size of hot room (which is a nice sized hot room by the way) but it has to do with how everything in your hot room has to get up to serving temp. to make for a good sauna, vs. one where the air is hot, but the walls are cold.

    The heater will heat the metal first, then some air, then some walls, and then benches, then some stones, and all this action takes time. Especially when everything is, as you say -20°f. So, you’ll have to report back. Even putting in a 18kW heater, you will incur the same thermal heat change challenge. And a big ass heater, say 15kW will cycle down too early. You will have a heater shut off, with air at about 180°f. and walls at 40°f. and that’s worse than a 8kW heater that is chugging along.

    Anyhow, don’t mean to be a buzz kill. You have to work with what you can do. Go electric, plow on, and sauna on!

  160. Thanks for the prompt response Glenn, much appreciated;

    Glad you seem aligned on the roof design;

    A:
    actually the “air gap between steel and roof plywood ” is just a membrane that would contain any roof leaks while offering some breathing if water makes it to the Plywood top or if condensation forms inside the roof cavity.

    My other options would be:

    1: use an impermeable membrane.
    I guess it would create even more condensation/mold/rot potential between the vapor foil and the membrane (IE: in the roof/insulation) as wood products need to be able to dry in at least one direction

    2: lay the corrugated steal directly on *treated* Plywood
    I’ll have all the required drip edges and foam closure strips but in case some water leaks, it should prevent rot; however since I’m very low sloped (read: I’ll slightly tip the sauna backwards with the ajustable supports to get something like a 1 or 2% slope) water would just wet the plywood and make the evaporation process longer than on the membrane (my perception is that the “perms” rating of the “Armor deck” (16) would facilitate the evaporation in the vented air space between the steel and membrane in case of a small leak.)

    Let me know if you would go for one of these alternative options or stick with my original plan.

    B:
    If I use some fencer wire around the base of the sauna to avoid critters sneaking under the floor, should I consider adding anything under the 2″ insulation or can I just leave it open ?

    BTW, thanks for the cement board / Quikrete vinyl option, sounds like a great approach.

    As for the heater, Once the sauna is completed, I guess I’ll start by enjoying the summer as It should be an easy task for the unit. Once winter comes back, I’ll do a lot of testing and report back;
    I’m into home automation as well, so the sauna will be rigged for remote starting/stopping as well as temperature/humidity monitoring. Hopefully it will be more a matter of a longer “pre heating” phase than not being able to bring the sauna up to a decent temperature; in the later case, I guess I’ll have to look for a more powerful unit….

    My wife wanted a large “all-glass” facade so this probably won’t help =)
    I have a full glass door 24×74 and two side windows 13×74;
    if needed, I’ll try to “insulate” the side windows for the colder months, or worst case, I’ll get rid of the windows and finish the walls (It does look good on plan though…)

    Thanks again Glenn,
    will take alot of pictures along the way so should be able to report back, hopefully with good news…. or lessons learned =)

    Cheers,

    Yann

  161. Hello, we are building a “birdhouse” styled sauna (4′ x 8′ cement pad, 7′ high, 7′ at widest point) using 3/4″ T&G cedar for all walls and roof, with steel roofing on top roof…(wood stove). My question is, is there any point to insulating the roof only, considering the roof is: 3/4″ T&G – 3/4″ spruce furring strips – steel roofing? What could we do? There is only 3/4″ space available, although we could increase the thickness of the furring strips to accommodate something?

    At the very least should we use tar paper or foil vapor barrier or something between the steel roofing and T&G cedar?????

    Thank you very much!

  162. For the roof. Yes, i’d insulate for sure. I would use 4×8 sheets of poly-iso sandwiched between the T&G and steel.

  163. Hello again! We are at the stage of getting ready to install the wood stove. The plan is to run double wall pipe 24″ up from stove, 90* elbow, 24″ out through back wall through a piece of sheet metal (3/4″ cedar wall – 10″ clearance from pipe to any wood), 90* elbow then vertically up 6′ (3′ above roof).

    For the exterior stove pipe is it necessary to use Class A pipe? Is it possible to use single wall? Thanks!

  164. Thanks for all the great info!

    I’m just looking for a little reassurance- I have an older cedar cabin that I am remodeling and had the external walls sprayfoamed before deciding to put a sauna in. The sauna will be in a corner so the two exterior walls have sprayfoam (with PT plywood and Tyvek exterior) and the two interior walls will be rock wool. I plan on using foil for a vapor barrier and will be extra liberal with the foil tape to make sure it is air tight. The unit I ordered is a Huum, so gets pretty hot (>200 F). So I’m hoping that with a good vapor barrier, any off gassing will go through the plywood and tyvek to the outdoor air- ALthough the sprayfoam is a vapor barrier itself so gas might get trapped between the foam and foil…. How worried would you be about this setup? Should I try to setup some kind of ventilation between the foam and foil to outside air or is this a setup for moisture issues? I’m already wondering if every time I go to relax in the sauna, I will be too preoccupied with the offgassing to enjoy it!

  165. Edit to the above comment:

    For clarification, it is only a small 5×6 corner room of the larger cabin that the sauna will be built into

  166. My opinion is that many get way worked up about spray foam and off gassing. I’ve done a lot of thermal modeling and feel comfy that the temp in the wall cavity is well below the service temp max for spray foam. And I sleep well at night with the foil vapor barrier well taped and sealed, as you detail. That’s just me. And I know others get worked up about such things, like drinking water in plastic bottles (including my wife) so for me, it’s a leap of faith towards whatever each of us think is reasonable.

  167. Hi and thank you for this resource.

    I am planning to build a free standing sauna in the garage/basement/lower level of an old Pennsylvania bank barn where I live. The space where the sauna will be going has an insulated ceiling, below grade stone foundation walls on 2-3 sides and insulated garage doors on the other side. Concrete floor w/vapor barrier. This space is approx. 1800 square feet and open.

    I have sourced some beautiful 2×12 (1.5″ nominal) Western Red Cedar Lumber. I am thinking of basically building a 4 sided box out of the cedar and then adding an insulated cedar ceiling and floor to be determined. Thinking I’ll cut 1/4″ grooves into the edges of the 2x12s and run splines in them to connect the edges then have staggered corners with a vertical corner post. Structurally this should be solid.

    This is a freestanding box and the unit will be indoors, protected from the elements, inside a large space (1800 sq. feet), and the 2x12s can theoretically act as both the interior and exterior walls I am thinking of forgoing any wall insulation or vapor barrier. The unit will be somewhere between 5-6′ wide, 7-8′ log and 7′ tall when complete. The space where the unit sits does not have big temp fluctuations and will not get below 35 degree F in the coldest weeks of winter.

    To me going this approach seems to make sense without wall insulation. I figure the cedar boards will shed their heat to the surrounding space which is actually ok and even desirable. What do you think? Talk me out of this….or is this “breathable” approach appropriate for this particular situation. If not what am I missing?

    Thank you!!

  168. Hi Jordan:

    I like the idea of using the “meat” of your 2×12’s for the wall. Lämpömassa enhancement vs. the thin tin can paneling that more budget builds are skinned with, on the inside.

    Now the only thing to concern with is your below grade stone foundation. If you ever get moisture through these exterior walls, that moisture will get trapped, and that’s the challenge with basement saunas with adjacent exterior walls. What I have done is glue and screw 2×2 firing strips to the foundation walls, 16″ oc, then rip 1 1/2″ rigid pink, set between these cavities, then foil, then t&g.

    I like creating a thermal envelope between the hot room and the stone walls, which suck heat.

    I hope this info helps you, Jordan.

  169. Thanks for this reply and what you say makes sense if I were building against the stone wall.

    However, I guess I should have mentioned that this sauna will essentially be a free-standing room/box within the larger space. Really the only place where the sauna will be in contact with anything other than itself is where it sits on the concrete floor and the plan is to insulate the floor and ceiling but not the 2×12 walls.

    So no contact with the stone walls. That said, you think this is cool without any wall insulation and just relying on the 2×12 cedar for both the interior and exterior sauna walls?

    Thank you again for this wonderful resource!

  170. I think this is totally cool, but the asterisks is that “it depends upon where you live.” Eg. barrel saunas in the Northwest USA. They “work” mainly because it doesn’t get colder than a well diggers you know what in that climate. So, 2x cedar stock can work as a wall. But take that sauna to Minnesota or Alaska, and, well, now it’s a new league of lämpömassa shortcomings, low convection/radiation ratios, and little engines that can’t.

    Hope this helps!

  171. has anyone tried to insulate without fiberglass insulation successfully? I’m trying to avoid the toxic material in a place designed to maintain optimal health

  172. Hi Luke:

    Yes, Rockwool batting insulation is a preferred material to fiberglass for this reason.

    The important thing is that whatever the insulating material, when we foil vapor barrier the inside of our walls, then seal off all seams and edges with foil tape, we create a barrier between hot room and walls. This is the ultimate security for good air in our hot rooms.

    And regarding no insulation, well, I have taken plenty of saunas (mainly in Finland) constructed without any insulation. A “hollow wall.” Exterior siding, studs with open cavity, then interior cladding, primarily spruce tongue and groove. The entire structure breathes. Living in Minnesota (brrr), I’m not a fan of this concept. No lämpömassa. A hearty wood sauna stove can beat back the lack of thermal resistance, but if going with an electric heater, well the vision in my mind is the little engine that can’t, trying to chug chug up the hill of good heat.

  173. What are your thoughts on timber framing a sauna? I am planning a build over the next two years. I am fortunate to have a very large cedar tree on my property that needs to be cut down. I would not normally consider timber framing a building like this, but I am going to be able to mill a whole bunch of lumber from this tree, so I could select timbers to be made for framing. Not sure about settling, bending twisting, shrinkage issues. Any advice on this would be much appreciated. Keep up the great work, I love this site!

  174. Hi Bryan,

    Glad you are digging saunatimes and all its intended goodness. “A very large cedar tree that needs to be cut down” to an authentic sauna enthusiast is like saying to an authentic fisherman enthusiast: “My grandfather owns a 300 acre private lake.”

    Instead of giving you all kinds of positive encouragement about how cedar will not settle, bend, twist or shrink, just start milling and when you’re almost done, give me your address and I’ll come by with my trailer and you can load some up for me.

    Good on you, Bryan. You are very much about to bark up a very good tree with this project. I am on your team on several levels.
    g.

  175. Hi Glenn! First off I would like to say thank you for providing all of this sauna building information to us for free! Your work is incredible. I recently purchased a rural off-grid cabin that has a detached wood fired sauna. I used it a couple times per week, before I encountered a problem. The builders of the sauna made quite a few mistakes. The first and largest mistake I noticed is that the stove was installed too close to the wall without a heat shield in place. So one day, as I was walking in to take a sauna, I noticed the insulation and T&G insulation were smoldering. I’m lucky to still have my sauna standing! I am redoing the entire hot room for this reason. I will be adding foil vapor barrier, which the previous builders did not use. But my main question here is what do you prefer to use for heat shield? From your recommendation, I will be using durarock cement board with a space behind it. What do you install over the durarock to make the visual appearance more appealing? Is there a specific type of tile that is heat resistant and works better than others? Thanks for your help!!

  176. Hi Paul..

    Glad SaunaTimes is helping you out! Bummer about your builder and mistakes. Not sure the wood stove you are using, but stove heat shields are a good place to start. And just to reiterate the sequence:

    Yes, if you can, I’d set it up as:
    1. studs, insulation.
    2. foil.
    3. cement board, screwed to wall.
    4. 1″ spacers
    5. 2nd layer of cement board, starting about 6″-1′ off the floor.

    The gap (between #4 and #5) is the real key, as the air cavity creates a thermal bridge between the mega hot and the wall.

    And as far as what to do to make the cement board appealing, there’s a couple options:
    1. vinyl cement coat the cement board.
    2. apply cultured stone.
    3. skin the cement board with metal.

    I’ve done all three and there is no easy front runner, just depends on how you want to roll.

    Hope this helps!

  177. Hi Glenn. I have purchased 8 by 12 tiny camper. It is insulated with Dow 3/4 residential styrofoam insulation. I was planning to bubble wrap over the 1/4 painted plywood walls and ceiling them install cedar. The floors are laminated wood over plywood. I plan to install concrete board over the floor. Leaving changing room as is . Your thoughts are appreciated.

  178. Hi Jon:

    Let me try to help by sharing how a typical sauna build rolls:
    framing
    insulation between joist cavities.
    foil vapor barrier.
    firing strips / air gap (generally preferred but sometimes questioned as to its full importance)
    t&g paneling.

    In your case, foil over painted plywood walls has me paused a bit. There’s condensation potential, maybe. I guess if it were me, i’d probably go with plywood, foil, firing strips, cedar. That seems best option to me.

    Thanks,
    g.

  179. Okay great. I won’t be concerned about styrofoam or the laminate flooring. Sounds like fine leave changing room as is. Appreciate your help. Jon.

  180. Thanks so much. Just verifying the styrofoam and laminate floor and changing room are okay. Appreciate your help.

    Thanks Jon.

  181. Some may get worked up about the non free range organic nature of styrofoam and laminate flooring, but these are in the cool down area where temps are not high, and so, yes, Jon, you’re ok here.

  182. Glenn, you have been a tremendous help. After some thought I have decided to take down the painted plywood and styrofoam in the hot room and put up old fashioned insulation. Cover with bubble wrap and then cedar. My plan is to lay concrete board over the laminate floor in hot room if okay and proceed to finish line.

    Thanks again for prompt response to all my questions.

    Jon

  183. Jon:

    You remind me of my son, where i’ll say “it’s up to you..” and let him stew on it, and ultimately he’ll make the right decision, which you did here.

    Pulling out the plywood and styrofoam is the better move.

    You will build this sauna one time, and be able to enjoy the goodness forever. Instead of foil bubble wrap, consider ordering foil vapor barrier from Amazon. It’s less coin and easier to work with.

  184. Well Glenn. Spent the day ripping walls and removing styrofoam. I did go ahead and pull up laminate flooring in hot room. Down to bare bones. Ready for action. You are right. After some contemplation I knew what I needed to do. Don’t want to second guess a bad decision forever.

    Thanks again for support. Really great to have someone who knows saunas. Take care

    Jon.

  185. Glenn, I am planning a basement sauna build. My basement has an unfinished ceiling (floor joists), and the ceiling is just 7′ tall. To make things more interesting, I would like to incorporate one of my basement windows into the sauna. The distance between the top of the window and the lowest part of the joists is 1.5 inches (a single 2×4). Meaning, my sauna ceiling will have to be a mere 1.5″ (the tongue and groove ceiling panels will add another half-inch or so, but that should be fine. It’s alright if the ceiling overlaps the grout around the window).

    Anyway… I was thinking the ceiling could consist of 1×4 boards with 3/4″ polyiso board slapped on top. Plus the tongue and groove inside. That’s a pretty weak R-value. Additionally, I could attach thicker polyiso boards to go between the joists. The 3/4″ polyiso would be right up against the bottom of the joists. Does that sound plausible? Or am I getting silly? Should I give up on the window?

  186. You can do this. Applying a thermal barrier between your ceiling rafters will help (no thermal bridging). The off gas police may frown upon poly-iso. I’d apply foil on the hot side of your rigid, then t&g.

  187. HI Glenn,

    Not necessarily related to insulation but reading through this made me think of another possible concern for me. I will be building my mobile sauna in Michigan this winter out in the Michigan elements. Do you think I should be concerned with board shrinkage in the cold? Do you think it would have a big impact if I install everything in the cold and then when it starts to warm up stuff would start to expand possibly causing joints to buckle?

  188. It’s hard to get it right. With paneling, cedar expands and contracts but not as intensely as other woods, my experience.

    Not this May seem a bit extreme, but I have taken saunas at the foil stage and have taken saunas with paneling cut just a 1/2” longer than finished dimensions. Lots of hot and steam and cool downs.

    It’s one way I’ve been able to “season” my paneling before final cuts and installation.

    This may work for you if you’re concerned about shrinking and expanding. Get your material really well acclimated before the finishing touches.

  189. Hi Glenn,
    Thank you for providing such a wealth of information for the DIY crowd. This thread persuaded me to use rock wool as opposed to the other options available. The idea of materials off-gassing in my sauna scares me. Do you think we should worry about light fixtures and electrical wires off-gassing? I am inclined to skip the lights in the hot room to achieve a totally natural environment, but wanted to know if there is any data to support one way or another.

  190. Brian:

    The data to support the safety of materials in high temp environment includes the service temp ratings listed by the manufacturers of these products. But I hear you, if you are suspect, there is no reason why you can’t go au natural everywhere in your hot room. I like it, and applaud your thinking! Candle window for light, etc.

  191. Okay, I’m actually more confused than before I read everything. I have your e-book, have framed my building, purchased my NIPPA, and will be installing roof and sheathing this week. Just confused about one thing now.
    Can you simplify it down to:

    –Unfaced insulation around the hot room, with foil barrier on inside and Tyvex/tarpaper on outside of entire building?
    –Faced insulation in the cool-down/dressing room, without poly OR unfaced with poly?
    And common wall? Is it the unfaced with poly on dressing room side, or no poly at all?

    Thanks for the clarification. I’m a Finn from the UP (grew up in the northland of MN) and now reside in Texas. This is my way of sharing one of the best parts of our culture with my kids and friends.

    Julie

  192. Hello – Tally here form the UK. I am building a super insulted cabin using a cassette build method (CNC machined cassettes which notch together) with 200mm insulation inside – Tyvek barrier and wriggly tin on the outside, vapor barrier and tape on the inside.

    The sauna is integrated in one corner and my question is – can I use the same insulation as the rest of the cabin – it is a recycled plastic material (ex. plastic bottles) called SupaSoft (https://supasoftinsulation.com/). It has a melt point of 180dec C (356deg F). My sauna materials supplier says he’s worried about it but the insulation supplier says that although they haven’t tested it specifically they think it will only gas off when actually ‘melting’ which hopefully it won’t do at 90deg C…

    Inside the sauna I am planning on using Rockwool, a vapour barrier then Finnish sauna cladding cladding. It’s the 200mm of SupaSoft insulation in the actual wall of the building I am worried about…

    I need to press go on the order and don’t know what to do 🙁

  193. Tally:

    Well, the conservative play will be to pull out the SupaSoft in the corner where the hot room will go, and do all Rockwool. You’ve got one shot at this, and it’s not a lot of material you’d be looking to replace, so if this were my project, i’d be using Rockwool throughout the walls, ceiling in your hot room. – a proven free range organic insulation, good in high temp sauna applications.

    Hope this helps!

  194. Hi Julie:

    Great that you’re sharing your culture down in TX!.

    yes, unfaced in the hot room, then foil on inside. Yes, Tyvec/tarpaper on outside of building. You got it.

    I would use unfaced also in cool down room and poly.

    Common wall is the only tricky part. Yes, unfaced, and foil on hot room side. I would poly the cool down side. And technically this sets up for a potential moisture trap in the common wall, but this won’t happen as you’ll tape your seams in hot room and cool down room.

  195. I’m considering using foil-faced insulation boards on the interior side of the 2×4 studs to create a continuous thermal break from the studs to the room (using the same John Manville Foil-faced Polyiso you reference that specs to 250 degrees Fahrenheit). Rockwool would fill the stud cavity. I’ve done this thermal break elsewhere in construction projects, but on exterior rainscreens with wood siding (so literally the opposite direction). There are some minor precautions to not over-tighten the furring strips/battens when screwing them in over the insulation (for installing the T&G later), but when done carefully with a string/level you can actually compensate for lumber crowns for straighter walls and easier T&G install. At any rate, any reasons why this would be a bad idea? I’m somewhat of an energy nerd, and thermal breaks are super important in passive house design, so I’d like to try it if it makes sense. I’d probably use 1-inch polyiso, but maybe even two.

  196. Hi Jeff!

    finally getting to this. It’s a delicate question/topic.

    I am so with you on the energy nerd! I have also done this thermal break Polyiso technique with projects and am a huge fan of the effectiveness of the thermal envelope. I converted our single stall attached garage to living space. I ran radiant wirsbo in the new cement floor pour, rigid underneath and hand dug around the perimeter to lay in 2″ rigid for thermal enveloping the warm slab. Fantastic. Then, yes, I did the John Manville Foil-faced Polyiso on walls and ceiling all the way around. Then sheetrock atop.

    I’m super proud of this project, it turned out insanely warm. (the former garage now living space has 3 of its 4 walls to the outside and roof too). And it gets colder than a well digger here in Minneapolis, but this room is the warmest room in our house. 1933 built.

    Anyhow, back to this as it relates to sauna. I’m right there with you. Given the temperature rating of foil faced Polyiso, i’m confident that if the John’s Manville testing results are right, there’d be no off gassing. There could be aufgussing, however, if you invite Germans to your sauna.

    I even called John’s Manville and talked with their tech team to confirm the 250°f. service temp rating. Now on the other side of the spectrum, there are the free range organic types who can put up a great argument about using only sustainable, natural materials in sauna. Not petroleum based foamy chemical stuff.

    I dunno.. just like with Bjork’s new album, I can go either way on this. What are you thinking?

  197. Hi Glenn,
    I’m building a sauna in the corner of my unfinished basement, against two exterior walls which are half concrete below grade and half wood above grade. I’ve seen your suggestion to fir out the wall with 2×2, but the wall is quite irregular (old, rough foundation, >1″ difference between concrete and wood paneling etc”).

    My solution was 1.5″ of foil-faced John Manville polyiso against the exterior walls, seams taped, then all four walls framed in with 2x4s. My original intent was to fill the stud cavities on the ceiling all four walls with mineral wool batts, then foil, then T&G. However, it seems I should avoid the double vapor barrier issue on the two exterior-facing walls, between the polyiso and foil.

    Should I skip the foil liner on the two walls framed against the polyiso, and only apply foil on the two interior-facing walls and ceiling? (Making sure the corner transitions between exterior & interior are well sealed). Should I skip the mineral wool on those two walls as well, leaving a 3.5″ air gap in the stud walls? Any suggestions are very much appreciated.

  198. It just occurred to me that any moisture that finds its way into the two walls framed against the polyiso can probably evaporate at the ends of the walls as well as the floor joists above. so there would be a way for some stray moisture to escape, as those two walls wouldn’t be entirely sealed in. Taking this into account, perhaps it might be OK to insulate all four wall cavities and ceiling with mineral wool and carefully seal the entire interior with foil? Thanks for a great website.

  199. Nick…

    I like where you are going. I appreciate all the concern about moisture in joist cavities, but your spirit above makes sense. Only guardrail I can see is the lack of free range organic nature of polyiso. But service temp rating of 250°f. I mean.. what’s wrong with that?

  200. Glenn,

    Thanks for all you do. I am starting a DIY Sauna build in my basement and been following closely.

    If I have a blank slate in the basement, and plenty of options. Is it better to put the Sauna in a situation where 1 or 2 walls are the poured concrete basement walls, or is it better to put it somewhere that all the walls are interior basement walls 2x4s not up against any concrete?

    At first.. I inexperiencedly thought the earth, and the concrete walls would be better for “insulating” but after reading many comments about how concrete walls draw moisture.. I’m not so sure anymore.

    Thank you!

  201. Andrew:
    Love your question, (and clearly asked!).

    Given your blank slate, it’s much better to set the sauna in a spot where all the walls are interior. This can give you two other advantages in that 1) you can purchase a kit, if you like, and 2) you have infinite possibility of hot room dimension/layout.

    Building a sauna with slab adjacent wall is doable, and you could do one wall to outside wall, as that may help your options. It can be tricky as you have to do it right, as there’s all kinds of moisture permeation possibilities, as you know and have read.

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