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!
Having grown up around the waters of Cayuga Lake, whose long finger touched the shores of my childhood stomping grounds, it is no wonder that our dreams often turned to things nautical. Since we first started taking saunas at Podunk in our early teens, the fantasy topic of floating saunas always came up. We loved swimming in the lake, but its waters are only warm enough to swim from the beginning of July to about mid- September. What a better way to extend that season than with a sauna? What a better way to sauna than not just near the water, but on the water (oh, but we did enjoy the naked runs to the creek!)
I’ve had an ongoing affair with boats: I have a love of canoes that goes back to my discovery of the Adirondack waterways which form an almost continuous route from civilization into the deep wilderness, and back; the caveat being that short carries were required.
I started making one years ago: a strip canoe affair. Not strip, as in naked, but “strip” as in thin bands of cedar, all joined and sandwiched between two epoxy and fiberglass layers. I never finished that boat; its progress was aborted midstream after I broke my collar bone in three during a trail running race. The unfinished shell still looms over my shop as a reminder, high up in a loft space. So, suffice it to say, that when a client approached me about building a much bigger boat, I had my hesitations about my luck with boats.
Mark initially wanted a beachfront sauna. Then zoning and other issues steered us to thinking of a floating sauna. My childhood fantasy! Granted it is not a new idea; in fact, there are several in Norway and other places. But on Cayuga Lake? This was to be a first. It made sense, in a fantastical way. He had ample dockage, and limited beach; he was willing to invest in the idea and take the risk, and he was a nice guy with just enough chutzpah to make it happen.
Client Mark and Family on Maiden Voyage of Sauna Boat, Cayuga Lake, New York.
The Design phase took over a year. It was a real challenge because this is not just carpentry but nautical engineering; precision was required, and my hand-drawn methodology needed some sharper pencils. Some 30 pages of drawings later and we were ready to build. We had a great fabricator for the frame, ladder-stair, and railing (Service Machine Tool in Elmira, NY) and some other great help along the way, but the whole thing—all 26 feet of it— was assembled in the shop. It was a challenge as the beast took over—floor to ceiling—and there was a lot of self doubt along the way to trip over.
I am not a boat builder so there was as much learning as doing, but we pulled it off—including the challenging work of four round cedar windows trimmed with real ship-salvage portholes.
There were a lot of other finicky details (I have come to understand that boat building is all finicky details). The biggest challenge was loading and transporting it on an oversize low-boy flatbed truck. At one point we had the 10,000-pound hulk levitating on three forklifts as the low-boy flatbed backed under it. The guys at Lansing Harbor Marina gave us confidence, especially after it passed its initial float test. After a few months of tweaking we took the maiden voyage, complete with a champagne toast.
The unique thing with our sauna is that it is a fully navigational boat with twin Electric motors and the sauna is fired with a gas fired heater and has 12-volt electric lights powered by a solar system. Ideally it will be used on a calm day when you can drift out to the middle of the lake, sauna, jump into the clear waters, cool off on the roof deck, and repeat until the fantasy has been satiated. Maybe even under the stars, or Northern Lights.
Thanks to everyone who helped make this possible, Especially Scarlet, who believed in the dream, and Mark and Karie, who supported it.
The old Sauna at Podunk had two rooms: a small dressing room and the larger hot room. The Old Nippa stove sat between them embedded in a masonry wall. Sitting on the benches we stared at the business end of the stove with its pile of rocks and the stove was tended from the dressing room.
This arrangement always made sense to me and is how I have been building my saunas for 30 years. I learned to weld in art school and set up my own studio soon after. Ozzie would send people my way for their stove repairs. After seeing how other stoves failed, I designed and started making my own stoves using much heavier plate on the top, where the heat would soften the thinner steel and typically lead to collapse under the weight of the rocks. I also kept to the external feed (thru-wall) and designed my stoves to be fired exclusively that way. As kids, we loved to pretend we could speak Finnish by stretching vowels and consonants together and making up Finnish sounding nicknames for each other. I called my stove the Lämpimämpi by combining Lemp and Memp. Finns will chuckle at this because it translates to: “warmer”.
I called my stove the Lämpimämpi by combining Lemp and Memp. Finns will chuckle at this because it translates to: “warmer”.
There are so many reasons for the external feed (thru-wall):
• The fire-tending, and ash debris are kept out of the hot room and you don’t have to tramp in and out with your boots on to tend the fire.
• Venting a small space can be complicated; a sauna stove requires significant combustion air which can create drafts or, worse, rob oxygen from the hot room. The external feed draws air from the dressing room or outside.
• Any stove front requires 36 inches of clearance to combustibles in front of it. This can’t be mitigated by heat shields. This severely limits the layout of the hot room. However, it is easy to get 3 feet in front of the stove in the dressing room.
• Any stove also requires a noncombustible hearth (stone) 18” in front of the stove. Hot ash and coals falling out the stove are a major source of fires. In a crowded and dark sauna room these hot coals can easily be overlooked, fall under duck boards, etc.
•A flickering flame to look at may be romantic but it is the soft heat off the rocks you want, not the searing radiant heat you get from sitting in front of a blazing fire. Typically, the fire may be almost out by the time the sauna is ready. The rocks should be the focal point. Also, following the 36-inch rule above, you can’t have the stove front facing the bathers, unless the sauna is excessively big.
• If you are providing a sauna experience for others, you can discreetly tend the fire without interrupting the bathers or invading their privacy.
•The external feed stove heats the dressing room just enough so that you can hang out and watch the fire while the sauna heats up.
Installing the external feed may seem daunting but it is not that difficult. A firewall with the requisite size opening will be required. This can be solid masonry, which will add thermal mass (and take longer to heat the sauna) or a hollow insulated firewall with steel studs and cement board, and tile or stone facing, or stucco over metal lathe (which I typically use). A metal sleeve will be provided with the stove to dress up this opening and provide further heat shielding. My Lämpimämpi stove has an integrated heat shield / rock basket that works with the wall opening so that fresh air coming in is heated directly by the stove and directed over the rocks, which is an advantage over simply having the rocks sit on top inside a steel box. As with any installation, all listed clearances need to be adhered to, but with this method, the stove will take up less space in the hot room and make for a cleaner presentation. For your next sauna, consider this traditional way of building it.
The emergence of Ai generated content has me worried about the future. People live, dream and work in the digital world and increasingly use Ai to create content, which seems to be the end, not the means to something tangible. As someone who has always worked with my hand’s it’s a mystery how something like Facebook gets valued in the Billions of dollars while those of us who actually produce products that are used in the physical realm rack up value only with each object we make.
There is a real disconnect from the process of making; a few computer clicks is all is takes to have a package show up at your door.
As a maker who has to rely on an efficient procurement process to keep my shop stocked, I get the impulse, but I also try to remember that there is always a person on the other end. For example, we buy vitahs that are from Estonia; I envision the worn hands that gather and bind the birch. I appreciate it when materials come with a hand written note of thanks or other human touches. The same search for human connection applies to communication. Each time I get an email inquiry I have my Ai filter humming- is this a real person? With the pandemic of spam and spoofing, I typically won’t even answer the phone unless know the caller and I spend way too much time deleting texts I don’t want. I occasionally scan the web to see what other sauna builders are doing; it means wading through a minefield of Ai generated crap. Not only do many sites not have any identifiable person attached to them but their ”product” is simply stolen images of other people’s work. I recently had to send a cease and desist notice to someone marketing a full array of my saunas as their own. Included were photos I took from inside my shop or in my backyard, complete with my sculptures. If someone is going to be so brazen, why not just steal my humanity and include a photo of me ? This is actually happening with deepfake videos that generate real-seeming talking heads of celebrities and public figures.
The good news is that building a sauna takes actual materials and real hands-on work. I can guarantee that each of the images on my web page are real, that the photos were taken by us in locations we were actually at and that they show just a sampling of the 150 or so saunas I have built.
The splinters I pull out of my hands on a near daily basis are real as well.
We are a family run business— basically just the two of us; there are no pushy sales reps or people working phones off site (as in another country). If we do answer your call it is only because the saws stopped running long enough for us to hear it ringing (which why we prefer email) and even though it may take a few days to get back to you, I guarantee it will be a real conversation. Hopefully, one that one leads to face to face meeting and enduring relationship. Saunas, after all, are all about human connections.
As you scan the web on a weekend morning, looking for that perfect sauna experience, be aware that only thing real thing may the aroma of the freshly brewed cup of coffee in your hand. If you don’t yet have your own sauna, find a friend who does or one of the many public saunas that are sprouting up, and immerse yourself in the physical realm, sans digital devices. Thankfully, Ai will never have anything to do with enjoying the blissful bodily experience of taking a real sauna!
One the endearing features of our saunas that falls under the rustic elegant motif that we employ is the use of stick hardware. These also fall under the category of Finnish Pragmatism that is an influence in my design; whereas superfluous embellishment is avoided and using what is at hand is always desired over spending for what you don’t really need. In my head I keep an inventory of all the random parts I have collected over the years that are stowed away in my shop and when a need arises I quickly do a mental scan and see if something in stock will do rather than going to a hardware store or jumping online. Likewise, I often resort to “natures hardware store” when I need things like door pulls and towel hooks.
It is amazing all the parts you can extricate from the intricate workings of tree.
The best is Hickory because of the way branches crook when they take off in a new direction, and it is very hard. After all, they make baseball bats out it. Recently we had to fell a Hickory so I salvaged all the door pulls and towel hooks I could from it.
wood-fired outdoor sauna
The tree will live on as it greets sauna users with a sturdy handshake each time they enter the sauna.
It’s the small personal touches and attention to detail that makes us proud of our work and makes our work fun and enjoyable. By avoiding the cold and the common place, we make each sauna as unique as its owners.
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.
