I always loved physics because it deals with the tangible effects of the forces of nature— the interactions between matter and energy—that explain the things that we feel or see on a daily basis. Specifically, I want to delve into the transfer of heat, which seems to be a hot topic in sauna forums.
There are three methods of heat transfer: conduction, convection, and radiation. In a sauna, (and everywhere else, unless you live on a planet at absolute zero: -460 °F) there are all three. Heat always goes from a warmer object to a cooler one, and a closed system is entropic, that is to say, if you sip too slowly, the ice will melt and your drink will eventually all be the same lukewarm room temperature. The transfer of heat is greater when the temperature difference (ΔT) is greater, and it slows over time, until the temperature equalizes in a system, which, for our study, includes not just inside the sauna, but the environment it sits in. Which is to say, no matter how well you insulate it, eventually the sauna will reach the ambient outdoor temp, unless, like a house (or a sauna in a house,) you keep the heat on. This is a factor in freestanding sauna design as we have to assume the starting point is anywhere from 0 to 100°F (unless it is fired up constantly) and the desired bathing temp is 180-220°F. In a house we are trying to hold the temp at about 70°, in the residential sauna, we need it to hold temp for a few hours, at the most.
Conduction is the transfer of heat from one solid or liquid to another by direct contact. You Grab a (foolishly installed) metal doorknob to the sauna that is either 200°F or 10°F, depending on the season, and which way you are going, and the heat rapidly conducts either to your hand or from it, with a resulting shriek. Same is true if someone pushes you against the hot stove as you try to leave the sauna, burning your butt to the point where sitting was impossible for two weeks, as happened to me once. This is conduction which we typically try to avoid in the sauna, but it happens. Less dense materials, like your towel, mitigate conduction, which is why we look for low density boards like cedar, not hardwood, for the benches, which would feel like a hot iron on your posterior.
Convection is the transfer of heat through the movement of fluids. It is in part driven by gravitational forces whereby warmer gasses or liquids are typically less dense and lighter and thus tend to rise as cooler ones sink. This create a convective loop as the heat is circulated to, say, the walls of the room, or you on the top bench, and then the air cools and falls, creating an endless loop. I say typically, because there is this oddball exception: water close to freezing gets less dense and thus freezes on the top of lake or pond, making hockey, ice plunging after a sauna round, and life on this planet, possible. If the movement of air is stopped, say by the fibers of mineral wool or two close layers of glass, it becomes an insulator. Air itself holds very little heat per volume-more than a thousand time less than water, whereas water holds twice the heat energy of granite and about the same as steel. A large volume of this dense, heat holding material is called a thermal mass, which can mitigate the fickle effects of convection, especially when the air is coming and going, by acting as reservoir of heat. That is why we try to keep the door closed in the sauna—all of the air convecting nice warmth around us is disturbed by the cold air rushing in to take it’s place. But that’s not so bad—as we actually want the fresh air—as long we have some thermal mass to mitigate the swings in temperature.
In home construction, the emphasis is on controlling convection: eliminating it inside wall cavities and not allowing warm air to escape from heated (conditioned) spaces, especially up high where it creates a chimney effect, whereby escaping warm air creates negative pressure and sucks in cold air from wherever it can. In a not-so-old house on a cold night, put your hand over the wall outlets—even on interior walls— and you will likely feel cold air being sucked in. More so if a you have a big cozy, romantic, fireplace with an actual chimney and a roaring fire, which feels great, but sucks the heat right out of house.
In a freestanding wood fired sauna, there will be leaks and cold air coming in. That’s ok because we want fresh air, as long as we control where it comes and goes. Air and steam will move the heat around but eventually it settles into strata: hot up high and cold down low. Air movement can help break up this layering of cold to hot, but it is difficult to control. Thus, the upper bench will always be hotter; unless you have an Aufgussmeister to move the heat around with his swirling towel dance.
The last method of heat transfer is Radiation. Sounds bad, like Chernobyl, but radiation is everywhere; all objects with a temperature above absolute zero emit thermal radiation, mostly in the infrared range that we can see with a special camera. At a certain point heat becomes light you can see and the color of the light corresponds exactly to a temperature. The dull red glow of a poker in the fire ( or the top of my sauna stove when I fire it hot) is 1200°F. The surface of the sun burns at 5772°K, which is the color of the sunlight we bask in on the beach. Fortunately, the sun appears relatively small, otherwise we would burn up instantly. We radiate as well; after getting sunburned, your skin will be hotter than the person next to you and will radiate heat to them. In fact all bodies, especially black bodies, which are not necessarily black, radiate and absorb heat, depending on which is hotter. The only things that are not black bodies are things like foil, which reflects most heat directed at it. Surface area and angle of incidence also matter; the more surface area and the more parallel two surfaces are, the more heat transfer. Temperature difference matters too: too much and the effect is intense, like when I pour bronze and have to stand an arms length away from the pot of molten metal, or stand on a subzero surface in winter and feel the heat being sucked from my body. Too little difference in temperature (ΔT), and radiation is hardly noticeable. Direction is also important. The fireplace heats our front but not our back. I have a story about a cold drizzly camping trip where all we could do to stay dry was to keep putting our jackets on backwards then forwards as we sat by the fire. And in all these situations it is aluminum foil that saves the day: as an apron to wear, a foil surface to stand on, or an emergency blanket over the shoulders. Foil blocks radiation, (but it does need an air gap, lest it become extremely conductive); without any barrier, heat, like light, radio waves and the rest of the electromagnetic spectrum can radiate millions of miles. Those episodes of Leave it to Beaver are still traveling through space.
In the sauna, radiation is really important as it creates this enveloping heat coming at us from everything hotter than 98°F. If the whole room- walls, benches and rocks, is 200° or more, we will feel the heat coming from each of those surfaces. Colder surfaces like a big window, or that guy that just got out of the cold plunge, will suck heat from us. Something too hot—like a blasting fire in a single wall stove pipe— will feel searing. In an electric sauna, the rocks need to cover the elements so we don’t see/feel the searing red heat. The much cooler—but still hot–rocks will then reradiate the softer heat. Foil, behind the cedar wall (or other wood), will reflect the heat back towards the cedar which will re-radiate towards the interior. The walls need to be just so hot. Radiation also mitigates the effect of the constantly changing air. The air may be cool, but the radiation of the hot surfaces will cut through the cold like the winter sun on your face. Speaking of, nothing like a full body sun-bath on a calm, freezing day to boost the sauna experience! The thermal mass mentioned above will continue to radiate heat even as the door is left open. Cool air swirling in will kill the radiation buzz for sure, but as soon as the door is closed that warm fuzzy feeling will come back.
So how does all of this daydreaming back to high school physics class inform how I build my saunas? A lot. I want the radiant heat off of the stove to work for me, warming me just so, like the sweet spot in the campfire where you should put your skewered marshmallow (but never do). I aim for a soft radiant heat, like a ΔT of a few hundred degrees at most (me: 98°, the rocks 400°,) but also omni-directional heat (which gets all the walls and benches up to 200° before taking a sauna) and not too intense (make sure the fire has died down and the stove pipe, if single wall, is not too hot). A big window is pretty to look out of, but not too big, as it will suck the heat away from you and a cold cascade of negative convection will sweep over the floor. Thermal mass is great, but not too much, because the sauna will take forever to heat up, and no one seems to have to time for the daylong ritual sauna used to be. I have my bathers all facing the rocks and typically the stove is fired from outside, so there is no worry about the intense (visible) radiant heat through the firebox glass door, which, as cozy as it sounds, may feel too much sitting around a hot campfire and is not the kind of heat you want in a sauna.
Recently I heard, in an online sauna forum, two seasoned sauna veterans saying you don’t want radiant heat in a sauna. I believe they misspoke. You don’t want high intensity radiant heat, but no radiant heat just is not possible, unless everything has reached a state of equilibrium. That is to say, you are as hot as the rocks, thus cooked like a goose (or the sauna is only 100°F). As long as you are cooler than the rocks, stove, walls and benches, heat will radiate to you. It is said that when you close your eyes in a good sauna, you cannot tell where the stove is.
How do we get there? Fire it hot to get the rocks and the whole sauna hot, but let the intense fire die down before getting in. Use radiant foil behind the wood (with an air gap) so the foil can reflect heat back into the wood, use a high rock capacity stove or heater to hold and radiate the heat, and make sure everyone can see the heater so the radiant heat—which travels as waves, like light— reaches everywhere.
You can always tell when there has been a really good sauna; everyone coming out looks so… radiant!
The latest sauna that we built is an indoor electric affair in a new addition that also holds a hot tub and pool, an enviable personal home spa combination. It has an ample window and large 10 kw Harvia Cilindro heater that should make the top bench a real hot spot. A feature of this sauna that I love is the view from that bench. Not just any view, but one that takes me back to my childhood.
The property is located on the eastern shoulder of Cayuga Lake, at a point where the land starts to dip dramatically downward to the long snaking shore. The slope is so steep here, that you don’t see the lake, only the opposite side, a little more than two miles away. Someone unfamiliar with our landscape might not even be aware that the longest of the Finger Lakes fills the glacial trough below. While the scene feels close, it is, in fact, a long journey away.
Most pronounced is Rice Hill: the old ski and sledding hill that, at one time had a rope tow run off of an old tractor motor. At the top of the hill is a warming hut and two shallow ice rinks, where many a hockey game was played. When we were too broke or didn’t have the ambition or means to go to the closest downhill ski area, we would go to Rice Hill and practice our ess turns. I also recall many tobogganing adventures; it was the kind of hill where serious injuries where the mark of a good run.
Just to the north was a ten-acre parcel my parents bought in the 1960’s with the dream of building a house. My dad designed it with all the meticulous detail he employed on his larger architectural projects. It was a three story modernist affair, with a flat roof, and cantilevered balconies that would have commanded a view across the lake to precisely where this sauna is.
In the early 70’s, things turned south for my Dad; there was a recession, he lost his job, increased his drinking, and the dream of the modernist masterpiece overlooking the lake was deflated like a balloon the cat played with. All we were left with was the model of the house my dad crafted out of mat board, with twigs as stand-ins for trees. Later, before my parents had to sell the property as a part a bankruptcy plan, I actually lived there in my tent after I finished Grad school and waited for my dreams to come to life.
I bring all of this up become of the associations of sauna with memory. So many of my clients, who are typically, like myself, aging baby boomers, tell me that they want a sauna because of the wonderful childhood memories they have of taking saunas. Perhaps their family has Finnish roots and they experienced summers in Finland, or they had a camp somewhere with a sauna. Like my experiences at Podunk, these childhood memories start to loom larger with age. Memory acts as a filter; the important things are retained and the trivial is set aside.
Landscape acts as a placeholder for memory. Living where I grew up, I constantly encounter places that stir memory. Working with my past literally out the window on this job, I was constantly reminded of my connection to this magical place in the heart of the Finger Lakes. Sauna is like a keystone in all of this.
Like so many saunas that I build, I dream for a moment; what if it was mine? But then, I hand over the sauna to the new owners so they can ponder their own dreams. In this case, the owner will be looking at the near view of the land he grew up on.
Traveling with one of our mobile saunas to the lake and delivering to new owners.
We have been building mobile saunas for the past 10 years! and because of their growing popularity and versatility, they now make up most of our business. Each unit is still handcrafted with many layers of details. The owners each get a unique product tailored to their specifications. It’s also an easy way to avoid zoning and permitting restrictions while avoiding the hassles of a site-built project. Investing in your dream sauna makes more sense if you know you can take it with you!
We hear from our customers all the time that their friends, family, and neighbors are as excited as they are to have a sauna!
mobile sauna, wood-fired sauna on a trailer
For many people, owning a sauna feels like a bare necessity during the winter months!
This recent sauna build is just steps from the owner’s historic New England homestead. Our saunas are designed to blend in with the home and environment. The classic details of this historic house are elegant rustic with dramatic rock outcroppings and a fire pit, making for a perfect gathering spot.Our wood-fired mobile saunas travel well and can be parked in beautiful placesScarlet by the lake ready for a cold plunge
Having a sauna at home is a life upgrade that is low-maintenance.
Our 5×8 ft Mobile Sauna parked in town with 100 gallon cold plunge tank.Interior of our larger mobile sauna. Our saunas can get as hot as you like with a large pile of rocks. We aim for 100°C / 212°F We build many mobile saunas in our shop in Ithaca, NY. Working in our 3000 square foot shop is more efficient than building on-site.Two sizes of mobile Saunas on display at our shop in Ithaca, NY
We offer building plans for DIY sauna builders or your local builder for one-time usage only. Thanks to our valued sauna plan customers, and the growing popularity of DIY sauna building, we have taken the opportunity to launch our new & improved mobile sauna building plans! Our sauna plans are 50+ pages and include detailed notes, drawings, photos, and material lists for a wood-fired 5’x8’/6.5’x10′. If you are thinking about purchasing our plans or building a sauna, we offer you an opportunity to build your own sauna using construction plans. Rob Licht has developed the best practices of sauna building with 30+ years of experience.
If the heart of the sauna is the kiuas, or sauna heater, then the soul is the rocks.
—Rob Licht
Every brand of heater I’ve installed has a different approach to the rocks. Some, like a wall hanging 6 or 8 kilowatt unit, use less- maybe 40 pounds, and some, like the Harvia Cilindro or Club heaters use more— up to 200 pounds. Wood burners vary too, Kuuma’s heater takes 150 pounds or so, but you have to provide your own. My own custom built stoves ( Lämpimämpi ) take a similar amount and like the Kuuma stove, the rocks are piled on top, mounded as high as you can. The Harvia Legend, an elegant wood burner from Finland, basically has a cage that surrounds the stove. By the time you are finished loading it, you won’t see the stove. There is even an optional cage that surrounds the stove-pipe to hold even more rocks.
The Finnish and Swedish heaters use Olivine Diabase or Peridotite, igneous rock found in Scandinavia. All of the rocks I have used that come with the electric heaters look the same: grayish chunks, some flatter, some chunky, all in the size range you’d find in a bag of potatoes. These are amazingly cheap considering that they come all the way from some quarry in Finland or Sweden. The UPS driver has stopped asking what is in the boxes (“yup, rocks”). Once I found a note in one from a young Swede hoping that I was happy with my rocks.
My favorite rocks are hand selected glacial erratics: various igneous granitic and metamorphic stones that I find in places where glacial melt-waters sluiced out potato sized rocks. (shown below) These are heavier and denser than our native shale, which is worthless for the sauna since it tends to explode when heated. They have a crystalline structure that you can usually see and a solidity and heft that is evident when you pick them and bang them together (they are not easy to chip.)
Lämpimämpi with Glacial Erratic RocksLämpimämpi with Jagged Olivine Diabase Rocks
A big consideration when selecting a heater is the rock capacity. The more rocks, the longer you should let your sauna heat up. Clients always ask:” how long should we let it heat up?” While I usually say “about 45 minutes” I really want to answer with the same sort of retort I give when someone asks how long will take for paint to dry (as long as it takes). The correct answer should always be: the sauna is ready when the rocks are hot enough to produce good löyly or steam—löyly is the soul of the sauna’s mysterious expression. And how hot is that? 450° Fahrenheit, more or less. If you want more steam, then you want more rocks.
Löyly is the soul of the sauna’s mysterious expression.
—Rob licht
If you expect several rounds or people coming and going, then more rocks are good. No one wants to pour water on the rocks expecting a stimulating burst of steam (but not too sharp) only to hear the fizzle of water barely boiling. That’s like trying to make an omelet in a pan that is barely hot. Disappointing, to say the least, but also bad for the heater. There’s Finnish saying that goes something like this: when you leave the sauna put another log on for the sauna elf or he will pee on your stove. Which means, don’t keep throwing water on the stove, especially at the end, if the rocks are not hot. The warm water will just sit on the steel stove and rust it. When you throw water on the rocks, it should all turn to steam and when you are done, the stove should dry quickly. When I finish eating my omelet, I rinse my cast iron skillet and let it dry over the flame, I don’t let it languish all wet in the dish rack; same principle applies to the sauna stove.
Jagged rocks will catch and momentarily hold more water, allowing less to get past the rocks and into the heater. The dense roundish rocks I use produce a nice soft steam but they have to be hot. I’m not so worried about water getting down to my stove, which is always very hot (and mine have 3/8” steel plate on top so they will never rust out). By the way, always use clean water and be wary of city water with chlorine or even country water with minerals or methane. We use only use filtered water for löyly at home. Spring water or distilled water might be prudent in some cases. Never use pool water or worse, let some disrespecting novice wring out their bathing suit over the rocks, as I have seen happen in a gym. Pleasant smells should emanate from the rocks. Special sauna oils or even Sauna Brew can be added to the löyly water for aromatic effect. Heck, we used to add beer to the water (cheap lager was always best).
Custom Sauna with Harvia Cilindro with carefully placed olivine rocks.
If your electric heater comes with the grey jagged olivine rocks, I suggest using them. They are selected to fit between and around the elements. When setting the stones first I rinse them, then I take care to arrange the rocks one by one. It can take me 45 minutes to fill a large capacity heater. For the Cilindro, I place large rocks at the bottom to hold up the others and then work a flattish layer against the cage wall. I then backfill the rocks against the elements with smaller and irregular rocks. On the smaller wall-mounted units I try to fit the flat stones between the elements without forcing or bending them. The goal is loosely fill the cage so there is good airflow and to completely conceal the elements. Lastly, I top off the heater with larger stones and try to arrange them so they will catch water and force it inward, not splashing it out. In Finland there might a “spirit stone” placed here: a special stone or stones from a favorite place (make sure it is igneous or metamorphic—test it in a campfire first if not sure). The top might even have a layer of more decorative stones, which in Finland you can buy from sauna dealers, but here in the US, I have only seem the same dull grey stones. The Harvia web page has a good section on stones and how to place them into the pillar.
Once you have the sauna going and have been using it for a year or so, expect the stones to settle and flake off. You should remove them, vacuum out the heater and replace the rocks loosely, discarding the broken ones and adding more if needed. Failure to do so will cause the heater to eventually overheat and lead to the elements failing.
Care in selecting and placing the rocks will ensure that your sauna…. Rocks!
It’s been a busy year at Rob Licht Custom Saunas and as the holidays approach it is a good time to look back everything that’s been accomplished, the hurdles we’ve gotten over, and to be thankful for the blessings we’ve received.
We started the year in the midst of the pandemic which made for some challenges but mostly the pandemic has meant a huge uptick in the sauna business as we all became more centered on home life and more reluctant to go out into public for things like gyms and saunas. Besides the several projects I have completed locally and around central New York I have fielded calls and emails from folks desperate to have their own saunas from as far away as Europe and Australia. I never set out to become a sauna expert, but here I am, 25 years into making them, and people are seeking out my knowledge from all over. In the process I feel like I have made many new friends. The global sauna community is alive and well. At the same time, due to the pandemic, I have mostly refrained from seeing all but my closest friends and family. It’s a strange new world but I am thankful to be connecting to so many people, if only on Zoom.
For my new friends I have designed and consulted on saunas from Maine to California— that has kept me busy when I was not getting my hands dirty. But whenever I can, I am working with my hands, either in my 3000 square foot shop, which I am ever grateful for, or I am on jobsites. It used to be that builders would simply stop in the winter and spend the dark months sitting around the woodstove reading back issues of Fine Homebuilding, but now we all seem to be out there in any weather. My cut off is 10° F; any colder and I want to be by a fire, in the sauna, preferably. Good gear helps; I’m especially grateful for my boot warmer and insulated pants.
Mobile Saunas showcased in the shop. Lansing, NY
Nothing I do is cookie cutter—I would die of boredom is life was too easy— thus the custom in my business moniker. This year I seemed to run the gamut of sauna permutations: First, a garage retrofit to a Yoga and sauna retreat, then a quiet walkout basement electric affair, then a classic one room wood burning sauna on a idyllic creek, then a more urban collaboration in Syracuse, followed by a tiny personal electric sauna in a bathroom, a rustic elegant wood burning retreat in the trees, and a distance job downstate. Currently I’m finishing up an electric sauna in a historic boathouse on Cayuga lake. I’m hoping to take it for a test drive, with a jump in the frigid water, by Christmas.
Garage Retrofit for Hot Yoga and SaunaClassic Creekside Sauna set 50 ft from swimming hole. Idyllic.Walkout Basement Electric Sauna BlissBackyard Sauna Design, Urban Collaboration in Syracuse, NYRustic elegant wood burning saunaCozy interior of a backyard sauna
In between all of these I have sold a few of my Lämpimämpi sauna stoves and many sauna plans. DIY interests, especially in the mobile saunas which are really big now. I get a kick out seeing my designs being brought to life by many different hands. It is also fun to see all of the other builders following in my steps. The more builders, the better. There is plenty of work to go around and I encourage anyone who wants to take the work seriously to pursue it with a passion. I did offer a sauna building class this year but had to cancel; Covid has thrown a wrench into a lot of plans. But stay tuned: perhaps 2022 will be the year.
Covid also threw into a wrench into the supply chain. We’ve all heard the phrase “supply chain disruption” by now. I bid jobs in the beginning of the year only see to prices on materials I quoted go up by 250%. Some materials simply vanished from the shelves. But now things have settled down and I also started ordering and stock piling materials well in advance. I can keep several saunas worth lumber and supplies in my big shop and insulate myself from some of the inflation—another reason to be grateful for the big work space.
Rob at the shop!
I’ve been working alone for most of the year, which actually suits me fine, especially with Covid lurking. Workers are hard to come by: not only are the skilled trades losing new blood, but, I think, the pandemic relief made a lot people lazy and unwilling to get off the couch. Scarlet, now my partner in everything, has been my greatest blessing. When she can escape childcare duties she has proven to be the hardest worker I could wish for. I could use a few more workers like her: eager to learn, unafraid of dust and dirt and willing to sweat. She also manages all of the marketing, so give her the kudos for the web media you see. The business feels like it wants to grow so if anyone is seriously interested in building saunas and wants to relocate to Ithaca, drop me a line.
For those of you lucky enough to have a sauna, I hope you get to celebrate the New Year in it for there is no better way to bring in the new and shake out the old. It’s been my tradition for four decades now and I hope to continue for four more.
Taking Sauna with Scarlet by the Lake, December 2021.
Lately I have been thinking about the application of the foil I use in my saunas as a radiant vapor barrier. Perhaps this is because it almost Christmas and I was thinking of how we decorated the tree each year. The final touch would be to drape foil tinsel over everything; our mother would have to constantly damp down our enthusiasm by reminding us to place it carefully on each branch, not to throw it.
This suspicious “sauna” foil is Aluminum-coated Plastic—upper working temperature of only 55-120° C.
There are tricks to using the foil but the first and most important step is to buy the right stuff. Like the tinsel we put on the tree, the foil may actually be aluminum-coated plastic— which you don’t want to use. That plastic is likely polyethylene which, if you look it up on the material specification sheet that every product has, it has an upper working temperature of 55-120° C, meaning it will likely melt at typical sauna temps. Sauna Foil, available from any of the familiar sauna suppliers, is aluminum foil on a kraft paper backing. I used to find it with fiberglass reinforcing thread, which is helpful because the stuff tears easily. Also helpful is 4 ft. rolls, rather than 3 ft so you can do a wall in 2 passes, but I have trouble finding this too. I recently tried a new supplier selling 4 ft rolls of “sauna” foil, but upon opening it had a suspicious plastic look to it. That night I put it in the sauna and within seconds it began to distort and curl up like the polyethylene I suspected it was made of. (See illustration above)
The second thing is to design the wall correctly. I read and see a lot of misinformation that touts using no air gap with foil. The air gap is essential. The foil works by reflecting radiant heat. All “black bodies” give off and absorb radiant heat that travels in a straight line from one hotter object to another cooler one, the hotter the body, the more heat it emits. The sauna rocks radiate a “soft” heat to you, the walls, and the benches, and that is why you want the sauna to be laid out so that everyone has a view of the rocks. The fire, if seen through a clear glass door, also radiates heat— but at a higher intensity. Too high for a comfortable sauna (but great for ambiance.) When that heat hits foil, it is reflected back into the room or the backside of the cedar—if there is an air gap of at least 1/2″. If it touches the backside of the cedar the foil— also a perfect conductor—pulls the heat away from the cedar and transfers to the wall space behind.
Air Gap. A Sauna Building Best Practice.
I’ve understood this for along time. The first semester of college I took a class: Solar Design and the Energy Efficient Home. We learned all about insulation, heat transfer and basic building skills. The first day of lab, where we were building a timber frame house, I was handed a Makita 12″ circular saw. My building career started right then and there.
With the web of misinformation out there I had to think of a way to illustrate this basic principle of thermodynamics that I learned my freshman year. So, one slow day in the shop I rigged up an experiment and photographed it. (see illustration below) I set up a section of cedar wall about 18″ from my infrared shop heater and fastened 2 pieces of foil to the back, one with a 3/4″ air gap, and one with no gap. After an hour the cedar was 250°F on the front—like it is often is in my sauna. The back of the cedar was 121° F, which is impressive by itself. The back of the foil with no gap was 115°F, meaning it was acting as a perfect conductor, and the back of the foil with an air gap was 71°F: room temp. The air gap was clearly making a difference, 45° in this case.
The thermodynamic experiment begins.
After an hour on cedar
back of cedar
back of foil, no gap
air gap makes big difference!
The foil is a perfect vapor barrier rated at zero perms— meaning no vapor moves through it. But unless you layer it properly, with insulation behind it, the moisture will condense on it, or the first cold surface it hits. Even in a perfect build, there might be cold spots in the insulation (typically about the size of a mouse hole), so there likely be some condensation, but not a problem if there is air movement. The air gap behind the cedar allows air to circulate around the cedar, removing any moisture and ensuring that the wood heats and dries evenly and remains stable. Heating one side of a board and wetting or cooling the other is how you make curved boat staves.
There are other tricks to using the foil- like unrolling it and re-rolling it foil-in, or using temporary magnets when working a commercial job with metal studs, but the key is to use care. Use plenty of hi-temp foil tape and patch tears as you go and work with a partner if possible.
I suppose you could build a sauna by putting a heater in a refrigerator box- but that would last about a day and be incredibly wasteful. Cedar touching foil won’t ruin your sauna and neither will plastic melting in the walls where you don’t see it. But if you are going to take the time and bear the expense of building a sauna, you might as well do it right and so it will last generations. I guess my mother was right: applying foil carefully and not just throwing it up is the way to work.
