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Community and Q&A

Insulating a Sauna

npcampbell | Posted in GBA Pro Help on

Hi there –

I live in Minnesota and there is a large sauna building community here. Specifically, mobile sauna’s are becoming increasingly popular. Currently, I have an order in from a reputable mobile sauna builder for one of these mobile sauna units. They are 7×12 (think sauna in a ice house trailer). The builder frames up the trailer, insulates it, and then provides all the cedar and materials to finish out the inside.

Here is my question: 

These units are insulated with 2 inches of closed cell spray foam. Admittedly, spray foam in a sauna makes me a bit nervous due to the lack of research around long term off gassing, especially at elevated temperatures (manufacturer states it is safe at 200+ degrees). I’ve been doing research and trying to come up with alternative solution for insulation. The conundrum is that the spray foam provides much needed rigidity and air seal to the trailer structure itself (it is framed with 16 oc aluminum framing with 090 aluminum sheet panels on the outside).

Here is my proposed solution for the wall envelope of the trailer to both reduce off-gasing concerns and ensure long-term rigidity of the unit: 090 aluminum sheeting>1 inch of closed cell spray foam>1 inch of Rockwool mineral wool batting (AFB), foil vapor barrier, T&G cedar.

This is basically a “flash and batt” method for a trailer (replacing 1 inch of the spray with 1 inch of mineral wool). My hope is that this will greatly reduce the heat that the spray foam is exposed to (which is my biggest concern with the SPF).

I am not a builder or architect. Does this envelop make sense? Would there be any of the common condensation issues with the 1 inch spray foam in MN winters (climate zone 6a). Again this is for a trailer that only has a wall thickness of 2 1/4 inch, so not sure if the dew point issue is relevant as with other “flash and batt” applications.

If I should avoid spray foam all together, what else would provide rigidity to the trailer (would also like to avoid pink rigid foam board)?

Any advice would be greatly appreciated by the sauna community here in Minnesota!


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Nick.

    A sauna is a special building and not one that I know too much about, but I have seen some construction details for them over the years and have used a few outdoor saunas. So, here are some thoughts based on what I know about their construction and the materials you are concerned with.

    I would like to see you avoid spray foam altogether (and XPS rigid foam). I'm not sure if you have to be concerned about off-gassing because of the sauna temps (though that's a risk I'd also want to avoid), but neither of these materials are climate-friendly and I don't think they are needed here.

    Because Saunas get intermittent use, you don't have to be too concerned with efficiency or wetting. I suggest a more vapor-open assembly (maybe mineral wool insulation) with a really good interior air barrier. Any wetting will have plenty of opportunity to dry when the sauna is not being used.

    I hope you get some replies from people who have built and operated an outdoor sauna long enough to know how it has performed.

  2. paul_iowa | | #2

    i live in decorah, iowa. I built a backyard sauna a few years back with non-faced r-13 fiberglass with paper backed rolled aluminum as the interior vapor barrier. all seams, tears and staples taped with aluminum hvac tape. no issues so far. the sauna/changing room is 6’x10’ and sauna itself is 6’x7’ with 6.5’ ceiling. it’s heated with a 6kw finlandia heater and takes all but 10 to 20 minutes to reach temp, depending how cold it is outside. don’t go with anything other than fiberglass or mineral wool, as foam would be totally overkill. and who’d want to spend time in a 165 degree off-gassing foam box anyway?

    1. charlie_sullivan | | #5

      This sounds like great advice to me. As for making it rigid, diagonal bracing will be stronger than foam. Depending on how it's done, diagonal bracing can make insulating with batts tricky, but there are solutions, generally metal straps in place of wood braces. But I don't know the construction of the trailer will enough to recommend anything specific.

  3. creativedestruction | | #3

    Closed cell foam needs to be a minimum of 2" to stop vapor diffusion and condensation potential. Insulating the inside of an aluminum can with anything other than 2" closed cell in Minnesota is very risky. Any tiny air leaks of the interior air barrier with a path to the aluminum will lead to condensation and smells. Aluminum is absolutely zero perms.

    You would have more options for insulation materials with a different 'shell'. Build it like an ice fishing shack -- stick framing on a flat bed trailer. Or alternatively, you could insulate the aluminum on the outside, like a can koozie. Forgive the bad metaphors...


    1. npcampbell | | #4

      Jason, this is basically an ice fishing shack (the builder made ice houses for 15 years before moving to saunas. It's aluminum frame built on steel trailer. He insists the spray foam adds needed rigidity to the trailer and normally uses 2 inches of spray foam. I hate the idea of foam so my compromise was 1 inch foam, 1 inch rockwool. But, I agree. I'm worries about condensation with this approach. Do you think there's be any issue with just going with 2 inches of semi-rigid rockwool all the way and no foam? Would not using spray have a significant negative impact on rigidity of structure that the builder claims?



      1. creativedestruction | | #6


        I honestly think there would be issues. That approach can only work if you can create enough of a drainage and air gap just inside of the aluminum to allow moisture out, ideally one with an airtight but permeable layer on the outer face of the mineral wool (like housewrap). You also need a perfect air barrier on the inside face, call it foil-faced mineral wool with taped seams. As soon as you tack on the cedar finish (to wood furring?), it's no longer airtight...

        I can't speak to the integrity of the structure this sauna builder puts up but it sounds like a beer can and he's probably not wrong that the spray foam helps a lot with rigidity.

        I like stick framed ice shacks myself. Wood insulates better ;)

        1. npcampbell | | #8

          Jason, thanks for your expertise. Still scratching my head here around alternatives to spray foam for this application. Another thought, would it be viable to use Rockwool and and not use a vapor barrier on inside of wall (replace it with a perforated radiant barrier)? Wouldn't this ensure that no moisture will get trapped in the walls since it would "dry" to the inside? Again, there will also be T&G cedar paneling on inside and adequate ventilation. Or is the concern that vapor filled air will still permeate through the Rockwool and condense on the inside of the aluminum sheeting?

          Thank you again for your thinking on this.

          1. creativedestruction | | #9

            "Or is the concern that vapor filled air will still permeate through the Rockwool and condense on the inside of the aluminum sheeting?"

            Bingo. It'll develop frost throughout winter and likely get smelly in the spring when it dries in. Aluminum and rockwool shouldn't develop mold but I wouldn't roll the dice.

            You could cover the 2" spray foam with Rockwool and be fine, assuming you can spare an inch or two and it solves the heat concern of the foam.

          2. npcampbell | | #10

            Got it. My thinking was with the internal temp getting to 180 - 200 F it would "dry" out any moisture left in the room after each use. But I suppose as the internal air temp drops (as sauna stove burns out), moisture could redevelop inside...

            I may actually go with 2 inches of aluminum-faced polyiso foam board / cut and placed between the wall cavity (and taped well). I know the R value drops in cold temps, but it seems to be a bit safer bet with the higher temps of the sauna than spray foam.

            Any issues with using polyiso foam indoors/inside the trailer? It has a max service temp of 250 F and if it's sealed along the studs, I shouldn't have to worry about any condensation, right?


          3. creativedestruction | | #11


            To make polyiso work, your interior airseal at the foil facing needs to be absolutely perfect. It's not much different than the foil faced mineral wool approach except the higher density of polyiso should make taping and sealing a bit easier. Know that it's risky.

            I'm not clear on how the interior finish is installed, but I wouldn't fasten anything through the polyiso if that's the approach you choose. Any air leaks at all will lead to condensation.

            Best of luck,

  4. AzurePower | | #7


  5. tundracycle | | #12

    @npcampbell, did you get your sauna built? By Eric? How'd it turn out?

    You should have 6 ACH of ventilation in your sauna and it should mix well throughout. If you don't have that then you should modify it so that you do. That should, I think, take care of most or all concerns about off-gassing?

    1. Jemari | | #18

      Super curious! Where did you find your 6ach recommendation???

      1. tundracycle | | #19

        I believe that's been the official recommendation in Finland since the 1960's (documented in the BIF-RTS 'RT Card' building systems). It's kind of unofficially been superseded by 10-12 l/s (20-25 CFM) per person.

        1. Jemari | | #20

          Thanks! I saw the same number (6ACH) in some FinLeo documentation as well. I am always dubious using numbers when I don't know where they came from. On the offgassing topic, the adhesive tape would be the biggest concern for me, and after that floor sealing products, but fortunately except where close to a heat source, they are probably within the intended service temperatures. There are high temp / low VOC aluminum tapes (Ex: Nashua Tape), although who knows what their definition of "low" is.

          1. tundracycle | | #21

            I think so long as the tape is hi-temp and adheres to the FF-PIR panels well then I don't think there'd be enough exposed area for off-gassing to be a concern. I could be wrong on that though.
            I'm not a fan of using any kind of sealing products in a sauna. Leave all exposed wood natural.

  6. mark_b_robinson | | #13

    Hi, I am stuffing with a similar design query and wondered how you got on?
    …For me I have a 28mm solid wood T&G roof, but don’t want off gassing of the bitumen roof covering getting back inside, so wanted to insulate to reduce temperature of wood roof, which then gets complicated with moisture etc which you were grappling with. What did you do in the end?

  7. tundracycle | | #14

    Mark, there's a good resource here:

    Finns (and pretty much everyone in Scandinavia) does framing & Insulation + foil vapor barrier + air gap + interior wood. Many also do exterior mineral wool. If the foil is installed and taped properly then it should do a decent job or preventing off-gassing from insulation / roofing materials getting in to the sauna but there is still a strong thought process to still avoid stuff like foam that will off-gas.

  8. plumb_bob | | #15

    I am in Northern BC, and I am a sauna enthusiast, and plan on building one soon myself.

    I just rented a barrel sauna, the construction is 2" cedar T&G with no VB or insulation. Metal roofing spaced off the T&G. Even at cold temperatures the small wood stove was able to overpower the lack of insulation and air barrier for a really nice hot sauna.

    I like this approach for several reasons: no off gassing of weird materials, major drying capacity, simple and cheap construction.

    Even with a structure larger than a barrel sauna, a used wood stove from a residential application would be able to throw off so much heat that the space should be piping hot.

    1. tundracycle | | #17

      The issue with the lack of insulation in barrels isn't overall heat but that people complain about their back being a lot colder than their front. In a normal rectangular sauna with higher benches the back of the bench is open so that hot air can flow down to counteract this but that apparently doesn't work in barrels.

  9. npcampbell | | #16


    Use a foil vapor barrier and Rockwool batts behind your cedar. You will be good to go with no offgass concern from the roofing material. Don't spend too much time or anxiety on this one, like I did!

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