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If our sauna walls could talk: insulate with vapor barrier or let them breathe?

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 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.

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!”

Sauna framing with Ken in New Zealand.

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135 thoughts on “If our sauna walls could talk: insulate with vapor barrier or let them breathe?”

  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. 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?

  10. 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/

  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. 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!

  23. 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.

  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 [email protected]

  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. 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?

  41. 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/

  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. 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?

  53. 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.

  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. 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

  56. 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.

  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. 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!

  60. 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.

  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. 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 :).

  64. 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?

  65. 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?

  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. 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!

  77. 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.

  78. 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?

  79. 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,!

  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. 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.

  83. 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.

  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. 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?

  90. 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

  91. 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?

  92. 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.

  93. 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!

  94. Hi Marc:

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

  95. 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?

  96. 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

  97. 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.

  98. 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!

  99. 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

  100. 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!

  101. 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?

  102. 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.

  103. 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.

  104. 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).

  105. 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.

  106. 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.

  107. 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.

  108. 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.

  109. 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

  110. 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″.

  111. 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

  112. 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!

  113. 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.

  114. 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!

  115. 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!

  116. 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!

  117. 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!

  118. 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?

  119. 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.

  120. 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

  121. 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.

  122. 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.

  123. 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.

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