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:
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 – 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
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
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
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.
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!