I love physics because it deals with the tangible effects of the forces of nature, the interactions between matter and energy that explain the things we feel or see daily. In this post, 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), all three types of transfer exist. Heat always goes from a warmer object to a cooler one, and closed systems are entropic, that is to say, if you sip too slowly, the ice will melt, and your cocktail will eventually be at a lukewarm room temperature. The transfer of heat is greater when the temperature difference, Delta T (ΔT), is greater. In addition, it slows over time, until the system’s temperature equalizes, which, for our study, includes not just inside the sauna, but the environment it sits in. Meaning, no matter how well you insulate it, the sauna will eventually reach the ambient outdoor temp, unless you keep the heat on as in a house (or a sauna in a house). This is a factor in freestanding sauna design. We must 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. Grab a (foolishly installed) metal sauna doorknob that is either 200°F leading into the hot room from the dressing room, or 10°F coming from outside into the dressing room (depending on the season), and the heat will rapidly conduct either to your hand or from it, followed by your shriek. Same is true if someone bumps you against the hot stove as you leave the sauna, burning your butt to the point where sitting is impossible for two weeks…as happened to me once. This form of conduction is typically avoided in the sauna, but it happens. Less dense materials, like your towel, mitigate conduction. This is why we look for low density boards like cedar, not hardwood, for the benches. Black walnut 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 gases or liquids, which are typically less dense and lighter, tend to rise while cooler ones sink. This creates a convective loop as the heat is circulated to the walls of the room, for instance, or to you on the top bench, at which point 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 a 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 times 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. By acting as a reservoir of heat, this mass can mitigate the fickle effects of convection, especially when the air is coming and going. This is why we try to keep the door closed in the sauna: The air convection that swirls invigorating heat around us is disturbed by the cold air rushing in to replace it. But that’s not so bad. We want some fresh air circulating and enough 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. This is especially necessary up high where warm air creates a chimney effect; whereby, escaping warm air creates negative pressure and draws 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 you have a big, cozy, romantic fireplace with an actual chimney and a roaring fire, which feels great but pulls the heat right out of your house.
In a freestanding wood-fired sauna, there will be leaks and cold air coming in. Again, 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 (−459.67 °F ) emit thermal radiation, mostly in the infrared range that we can see with a special camera. At a certain point, heat becomes visible light, and the color of the light corresponds to a specific temperature. The dark red glow of a poker in the fire (or the top of my sauna stove when I fire it hot) is 1200°F (these are specific colors-blacksmiths, for example, will have a color chart on their shop wall from dull red to bright yellow). The surface of the sun burns at 5772° Kelvin, which is the color of the sunlight we bask in on the beach. Fortunately, the sun is far away and appears relatively small; otherwise, we would burn up instantly. The human body radiates heat 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 bodies1 (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 as well. Too much difference and the effect is intense, like when I pour bronze and have to stand an arm’s length away from the pot of molten metal, or when I 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 when all my companions and I could do to stay dry was to keep putting our jackets on backward then forward 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 needs 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 from your parent’s childhood are still traveling through space.
In the sauna, radiation is crucial as it creates an enveloping heat that comes at the bather from everything hotter than 98°F (body temperature). If the whole room—walls, benches and rocks—is 200°F 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 heating elements, so we don’t see/feel the searing red heat. The much cooler, but still hot rocks will then re-radiate the softer heat. Foil behind a cedar wall (or other wood) will reflect interior heat leaving the building back toward the cedar which will re-radiate toward 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 which, there’s 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 if 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 this daydreaming back to high school physics class inform how I build my saunas? A lot. I want the radiant heat off the stove to work for the bathers, warming them just so, like the sweet spot in the campfire where campers should roast a skewered marshmallow (but never do). I aim for a soft radiant heat with a ΔT of a few hundred degrees at most (the bather: 98°F, the rocks: 400°F); an omni-directional heat, which gets all the walls and benches up to 200°F before sauna time; and a not-too-intense heat. (Make sure the fire has died down, and the stove pipe, if single wall, is not too hot.) A big window is pleasant to look through, but it must not be too large, as it will suck the heat away from bathers, and a cold cascade of negative convection will sweep over the floor. Thermal mass is great, but again it must not be too substantial because the sauna will take forever to heat up, and no one seems to have the time for a daylong sauna ritual as in the days of old.
I have my bathers facing the rocks. Typically, the stove is fired from outside, so there is no worry about the intense (visible) radiant heat through the firebox glass door. As cozy as that sounds, it may feel too much like sitting around a hot campfire, and that is not the quality of heat you want in a sauna.
Recently, in an online sauna forum, I read two seasoned sauna veterans stating, “you don’t want radiant heat in a sauna.” I believe they misspoke. High intensity radiant heat does not belong in a sauna, but a lack of radiant heat is only possible if all surfaces, bodies of mass, and liquids have reached a state of equilibrium. That is to say, equilibrium can be reached in a sauna of 100°F or when it is as hot as the rocks, in which case, the bather is cooked like a goose. As long as the bather is cooler than the rocks, stove, walls, and benches, heat will radiate to them. 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? Install radiant foil behind the wood walls (with an air gap) so the foil can reflect heat back into the wood and back into the sauna; use a high-rock-capacity stove or heater (thermal mass) to hold and radiate the heat; fire the kiuas (stove) hot to get the rocks and the whole sauna deeply heated, but let the intense fire die down before bathing; and make sure everyone faces the stove, so the radiant heat (which travels as waves, like light) reaches everywhere.
You can always tell when a sauna has good löyly; everyone coming out looks so… radiant!
To save it the old Sauna at Podunk, it had to be taken down. The squirrels had invaded and filled the dressing room with a cache of black walnuts. The building was slowly sinking into the earth, and the safety of the chimney—a heavy cast cement affair, supported in the ceiling by a rusty homemade contraption—was questionable. The gaping mouth of the woodstove, its door rusted open, was set in a permanent expression of whoa!
If the sauna was ever to make löyly again, work would have to be done. So a month ago, after careful consideration and much debate, Scarlet and I joined members of the Heila family for a day of deconstruction.
As you may recall from earlier posts about Podunk (Sauna Time, Sauna Ritual,Homecoming, Back to Podunk), this is the ninety year old sauna where many of us locals were initiated into the joys of sauna during the heyday of the 70s when the Podunk Ski Center was a mecca for Nordic Skiing and all things Finnish. The structure’s simple rustic character, which addressed the basic functionality of the sauna with what I call Finnish pragmatism, is the inspiration behind much of my sauna building. The demolition would give me the chance to dissect it and uncover some secrets of its original design.
We always thought it was the perfect sauna: Hot but airy, it made good löyly and was roomy enough for an intimate crowd of eight.
What I did not know was how the materials related to its function: how it heated up so well, how it held a good Löyly and never felt stuffy, and why it never burned down. Aesthetics aside, these are essential components to a well functioning sauna. We often debated whether it had any insulation at all, so I was especially curious about that.
We gathered on a drizzly morning with a chill in the air. Ironically, it was a perfect day for sauna. Our plan was to document the existing structure and take it down methodically, saving what we could and carting the rest away. Eventually, the structure will be rebuilt on the same site, the design as close to the original as possible. We proceeded quickly, each of us attacking a specific area. Beloved details, like the doors and little shelves in the dressing room, were labeled, wrapped, and safely stored. The repurposed barn siding was carefully removed plank by plank, and the whole front facade was Sawzalled off and preserved. As the layers were peeled back, we discovered not only that there had been several incarnations to the structure but we revealed answers to some of the questions I had been pondering. There were several surprises.
As the walls were removed from the outside in, we uncovered many layers and each wall was different. On the east wall, under the vertical reclaimed barn boards (installed in the ’70s?) was a layer of Inselbric, the ubiquitous and horribly ugly asphalt siding that was used starting in the ’30’s until aluminum siding became popular. The product was easy to use and durable and is still found on many economy (or as my Dad would call it: “Early American Poverty”) style homes dating between 1930 and 1960. This was over a layer of horizontal 1×6 pine boards, loosely spaced, which went around most of the building. The big surprise was under these boards: flattened cardboard boxes, several layers deep, between rough-sawn, vertical framing members about two feet on center.
The cardboard was in good shape and the labels were easily read: cereal case boxes from Wheaties, Corn Flakes, and others. This was the insulation we all had wondered about!
A web search of the logo style led to verification of the sauna’s 1935 date of construction. Vertical boards were interspersed with the cardboard with no apparent purpose. Was this to add thermal mass to the walls? The interior surface was initially all Beaverboard (an early fiber board) that had been covered with a thin veneer of plaster (real plaster, not joint compound) which was painted. This was akin to the plaster and tile block sauna of Van Buskirk Gulf I’ve written about in a previous blog post. The skimcoated Beaverboard provided a vapor resistant barrier that held the Löyly steam for the right amount of time. Later, in the 1970s, this barrier was covered with 1×6, tongue-and-groove, knotty pine. Given the current popular obsession with cedar interior sauna walls, I wonder if a more authentic sauna might be simply plaster with wooden benches and back rests? The plaster and paint layers (probably lead paint) were vapor semi-impermeable and thus capable of holding some of the moisture. Surely all the outer layers in the walls were breathable; that is, the allowed vapor to easily escape and not collect as condensation. This is a very important consideration in any kind of construction. But one corner post had signs of severe rot. Did the plaster layer crack here and allow moisture to saturate the wood? That must have opened the door for a colony of carpenter ants that moved in.
I also noticed that other than the entire building sinking into the earth, the walls were structurally sound. So much so that when Tom hooked up the tractor to pull the north wall off, the whole remaining building (already missing its east and south wall) simply hopped along the foundation slab behind the tractor, taking the chimney with it and sending everyone into fits of laughter. All those random layers of heavy boards were keeping things together. It’s not a recommended practice, but sometimes just heaping layers of wood into a structure creates enough redundancy to make it solid. I prefer the more efficient approach of building more with less.
The ceiling repeated the wall construction: plastered Beaverboard covered with pine. The tiny attic space was filled with a layer of cellulose—peppered with rodent droppings, and walnuts—empty boxes of rat poison, and a few old bottles here and there (which probably once contained hooch). One attractive bottle was verified as being from 1938 by its unique design. Probably teenagers hiding their stash after a sauna, but quite possibly, offerings to the sauna Gods to protect the structure from burning down.
As for fire safety, it was barely existent. It was a miracle that the sauna didn’t burn down. There was charred wood throughout the attic, especially around the iron chimney supports.
Again, there were a lot of heavy boards laid across the ceiling joists, which seemed to have no structural significance, perhaps only adding thermal mass or insulation. The roof rafters were so heavy, and the roof so strong, that after it was lying on the ground like a low pup tent, Tom had to drive the tractor over it to break it apart. The metal, standing seam roof, with its many coats of black tar, was in surprisingly good shape, but leakage was occurring where the heavy, cast cement chimney penetrated it. The stove below, welded by me in the 1990s, was so rusted it was deemed scrap.
The cement floor had sloped to a drain but was cracked and broken. The original cement pour seemed hodgepodge and lacked any rebar. Woodchucks had tunneled voids underneath it. The drain had allowed bathing—something the early Finnish farmers needed as the house probably lacked plumbing. The floor will be replaced with an edge-thickened slab as the foundation—with a solid gravel base over undisturbed earth reinforcement with steel.
I consider bathing an essential part of the sauna experience. It is a function of the sauna that informs my designs.
Perhaps one component why the sauna felt so good was all the brick work around the face of the stove—the stove was fired from the dressing room—a traditional design I frequently use. (External Feed Sauna >) This added about a thousand pounds of thermal mass to the hot room. Thermal mass holds heat and radiates it back into the room, very desirable. But it also means it will take longer to heat up. I typically use a lightweight fire wall (between hot room and dressing room) so the sauna will heat quickly and to lessen the load on the building structure. Perhaps I should rethink that and revert to the solid masonry I started building with in the ‘90s. Ironically, the brick work at Podunk was added in the ’70s. The old Finns in our region commonly relied on asbestos board for fire protection.
By the end of the day, we had a pile of barn boards and other parts stacked and labeled in the old ski lodge, and a dumpster overflowed with the rest. Although most of the sauna was discarded, the lessons learned will live on in the saunas I continue to build. Next year, we will rebuild Podunk with modern efficiency but in the same basic footprint as the original. Hopefully, the entire facade will be replaced and the lilac tree where the sauna bell hung replanted. We’ll probably skip the lead paint and asbestos board and use a modern, UL listed chimney support in lieu of the original homemade rig. Fire safety will be based on science, not luck (or sauna Gods). The walls will be lined with cedar over foil (with an air gap!), and the functionality will be the same, and hopefully, better.
Family and friends will gather there to sweat and bathe and run naked to the creek for generations to come.
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 is almost Christmas, and I was thinking of how my family 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 131-248° F (55-120° C). This product compares to the ubiquitous foil “bubblewrap” people and is not to be used inside your sauna walls.
There are tricks to using radiant vapor barrier 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, has an upper working temperature of 131-248° F (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. Four foot rolls, rather than three foot are helpful so you can do a wall in two passes, but I have trouble finding that width.
I recently tried a new supplier selling four foot 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 trick is to design the wall correctly. I read and see a lot of misinformation that touts using no air gap with foil. This is wrong. The air gap is essential. The foil works by reflecting radiant heat. All black bodies1 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). With an air gap of at least a half inch, when the heat hits foil, it is reflected back into the room or the backside of the cedar. Because the foil is also a perfect conductor, if it touches the back side of the cedar (as will happen with no air gap), it pulls heat away from the cedar and transfers it to the wall space behind the foil air gap.
Air gap. A sauna building best practice.
I’ve understood this thermodynamic principle for a long time. I took a class called Solar Design and the Energy Efficient Home in my first semester of college. We learned all about insulation, heat transfer, and basic building skills. The first day of lab, wherein we built 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. 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 two 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 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 a 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 will be some condensation, but this is 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: unrolling it and rerolling it foil-in, 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 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.
Increasingly, I hear from clients who want a sauna as a way to enhance their hot yoga (Bikram) practice. It’s a perfect pairing: What better way to follow up (or warm up for) a hot yoga session than with an even hotter sauna!
Recently, a couple asked me to convert an old, dingy, freestanding cinder block garage into a sauna/hot yoga studio. First, I made sure the cinderblock wall was stable and did some minimal repairs. Then I isolated the block wall from the warm, humid space by adhering expanded polystyrene (XPS) foam board to the walls. This was critical as to prevent moist air from hitting the cold cinder blocks and condensing. Next, I framed in the space, insulated the walls with mineral wool batts, and finished the yoga space with drywall and the sauna with cedar (with the requisite radiant sauna foil layer and air gap). New windows replaced the old; dramatic deep window recesses were a result of the thick walls. Bamboo flooring over floating sleepers over foam board created just the right bounce for the yoga space, while the sauna was fitted with traditional duck boards. LED lighting added just the right ambiance. The heart of this sauna is the Harvia Cilindro heater with it’s two hundred pound rock capacity.
Amazingly, the old building was plumb and square—the original masons did a good job. Fighting an out-of-square space is the bane of all renovators.
A sanctuary just a step away from home.
This project was a complete transformation for this building (see below), turning it from a creaky, old, under-utilized garage into a revitalized space for self-transformation. Make an inventory of the neglected spaces on your property awaiting transformation and give me a call!
Operable vent with slider doorFloating Bench, Hot Yoga Studio
You’ve probably heard that I’ve spent a lot of time in and around the saunas. But another hot spot I’ve spent a lot of time around is kilns. Specifically, foundry kilns and ceramic kilns. Unsurprisingly, there is a strong relationship between the two, as they both involve getting things hot. In the lost wax casting process, investment or ceramic shell molds are heated to roughly 1500° F. The extreme heat burns off the wax original, and thus, the lost wax of lost wax casting. This can take hours or even days depending on the mold type and size. A ceramic kiln can get much hotter, up to 3000° F. That is hot enough to melt steel and many other metals.
I learned how to do bronze casting in art school. It is an ancient process, and my classmates and I did it pretty much the same way that it was done thousands of years ago. We learned to determine how hot things were by using our senses. All objects emit radiation when heated but at about 1100-1300° radiation becomes visible. Peering into a hot kiln (safety glasses strongly suggested) is like looking at another world, perhaps on some gaseous alien planet. The blast of heat through the spy-hole is like a ray gun. Solid objects become transparent. Heat and light become one; the heated molds don’t reflect light but emit light. We rarely used pyrometers (hi-temp thermometers), and when we did, it was only to affirm what our senses were telling us. We recorded the smells of things burning off. When the smells were gone, the molds were clean and ready to accept the molten bronze.
When a kiln is loaded, there is always discussion about the hot spots—certain delicate molds need to avoid the highest heat while larger molds might need it more. There is always conjecture about how the heat circulates; a whole aspect of kiln building is dedicated to controlling the flow of heat within the kiln. Some of this conjecture is borne out in the results of a firing—whether things fire correctly or not. Ceramicists use cones: small tapering forms that bend at specific temperatures. After a firing, these devices will give a true telling of how the firing went. But despite the science, there is still a lot of mystery and art to the process, so much so that a firing of a large kiln can take on a ritualistic feeling. Staying up late to tend the kiln (as is done with wood fired and other non automated kilns), drinking beer, and heating up pizza on its surface add to the aura.
Thinking of all of this casting lore makes me think of sauna. Both processes have been done pretty much the same way for millennia, involving community and an aura of ritual. Both focus on fire and heat, and even as well studied and commonly practiced as they both are, there is still a bit of mystery involved in each process.
A kiln is like a sauna on steroids. The heat is so amplified that its flow and effects are unmistakable. Observing a kiln is a lesson in thermodynamics. In the sauna building culture, there is a lot of banter about how to best heat, insulate, and vent a sauna. Yet, all of it is conjecture, based on theory, until one sits in a sauna and feels the heat radiating off the rocks and the wave of löyly hitting the sensitive tips of your ears.
When I design a sauna, I draw from my years of kiln experience. I think of the heat as a visceral substance, almost visible, as in a kiln. I relish using my senses to discern quality rather than depending on technology. Even if the sauna is electric with a digital control panel, I rely on feeling, not the number on the display. I imagine the flow of heat like the way it flows in a kiln. My foundry experience has informed my understanding of sauna in ways that are hard to describe, but suffice it to say that I have always been drawn to fire and to the mysteries that it holds.
A lot of building science is pretty theoretical. No matter how much research you do, at the end of the job, most of the work is hidden in the walls. Unless you come back to do renovations or worse, get a dreaded call back for something gone seriously wrong, you rarely get the opportunity to see how your work performs. I am not talking about cosmetic details like nail holes that don’t get filled or rough edges that didn’t get sanded. I’m talking about how well materials hold up to the heat or how well you have managed moisture movement through the walls, either as precipitation working its way in, or the more mysterious way that water vapor works its way out (or inwards in some climates).
This moisture is driven by vapor pressure, which can drive water molecules through most any material given the proper humidity and heat differentials—something a sauna has a lot of.
I learned about vapor pressure when I realized that my hollow steel yard sculptures inexplicably filled up with water. My welds are very solid and water-tight, but somehow, moisture was penetrating the steel, condensing, and not getting out. Cutting holes in the bottoms to let trapped water out solved that problem. There’s a bit of molecular science involved here, but suffice it to say, that vapor pressure is very strong—strong enough that when I throw water on the hot rocks, my sauna door pops open as if the löyly has scared a ghost out of hiding.
Thinking about all of this has left me wondering what is happening in my sauna walls? Am I doing a good job? Is the insulation holding up? Is water getting trapped?
Yesterday, I had to do some retrofitting on the first mobile sauna I built in 2013. I exchanged the Scandia gas heater for a wood burner and got to peer into the dark interior of the walls. This is a sauna that has seen heavy and very hot (200°+) usage. The walls were built with cedar inside and out with only a 1″ layer of foil-faced polyioscyanurate foam board between the studs.
Here is what I found:
There was no damage from trapped moisture, and the foam board looked as good as new; the foil facing was still shiny!
There was some hi-temp fiber-fax insulation used around the gas heater. A rodent had gotten into this (despite my filling gaps around the gas line with steel wool) and made a stinky little nest.
So, this confirmed my use of the polyisocyanurate board, which has a service temperature of 250°F, and verified my disdain of fiberglass-type materials (rodents love it).
Polyisocyanurate board, original state.Fiberfax mouse nest.Melted EXP (expanded polysytrene) foam. Do not use!
On my desk, I have a piece of expanded polysytrene foam (EXP) I pulled out of a failed sauna I was asked to repair. It looks like one of my steel sculptures from my Landform series—a flowing, green landscape (of melted plastic). Its service temp is listed as 150° F. All materials have material data sheets, usually available on the manufacturers web pages. I consult these whenever I am unsure about materials, especially given that the extremes of the sauna are like the extremes NASA engineers have to deal with.
Clearly, there is a correlation between science and reality, even if it is happening unseen inside the walls of your sauna. So when choosing materials, listen to the science, learn from observation, and don’t just buy the cheapest materials or use only the easiest approach.
Consult with the experts. Sometimes, there is more to it than meets the eye.
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