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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84 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.

  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 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. 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.

    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.

    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!

  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.

  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.

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