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Can I use plastic foam to insulate my sauna?

Can I use plastic foam to insulate my sauna?

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

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

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

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

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

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

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

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

Expanded Polystyrene - EPS - Thermal Insulation

EPS – Expanded Polystyrene Foam

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

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

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

XPS – Extruded Polystyrene Foam Panels

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

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

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

Polyisocyanurate Foam Board

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

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

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

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

Facts About Spray Foam Insulation | Ecotelligent Homes

Polyurethane Spray Foam

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

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

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

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

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

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

In closing

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

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

unreadable excerpt from product sheet for Johns Manville polyiso.

Link to Johns Manville Polyiso product sheet here.

Those suspect of Poly-iso can be supported by The Sauna Twins at Finnmark Sauna. Please listen our Sauna Talk here. Flammability is indeed an issue of consideration. Is there a magic product for insulating our saunas? Do these choices of material affect indoor air emissions?

Atlas EnergyShield and GAF EnergyGaurd offer halogen-free polyiso. However, hard to source….

More to come!

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36 thoughts on “Can I use plastic foam to insulate my sauna?”

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

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

    G.A
    Clark, Colorado

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

    Nick

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

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

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

    Thanks for your assistance and have a good day.

    Tim

  5. Hi Tim:

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

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

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

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

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

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

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

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

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

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

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

    I’ll post some insulation pics on Instagram @saunasbyrob

    Happy New Year everyone!

  9. Hi, everyone!

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

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

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

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

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

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

    THANK YOU & HAPPY NEW YEAR!

  10. Hi Mark:

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

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

    3. yes.

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

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

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

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

  12. Hi Mark:

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

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

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

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

    G.A

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

    Hope this helps!

    Nick

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

    G.A
    Clark, Colorado

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

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

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

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

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

    Please let me know your thoughts.

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

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

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

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

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

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

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

  22. Hi Glenn

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

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

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

    Thanks for this great resource.

  23. Hi there

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

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

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

    Thanks!

  24. Hi Danny:

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

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

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

  25. Hi there!

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

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

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

    Much thanks!

  26. Hi Glenn,

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

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

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

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

  27. Hi Carol:

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

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

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

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

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

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

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

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

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

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

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

  28. Glenn,

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

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

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

  29. Glenn

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

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

    Any insights?

  30. Thanks for all the great info!

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

  31. Edit to the above comment:

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

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

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