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 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.
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.
I have always been fascinated by fire. There is a mystery to it that even science cannot unravel. It is more like the fourth state of matter—plasma—than the familiar trio of liquid, gas and solid. I have played with and studied fire since I was a kid, sometimes barely avoiding serious trouble, other times under the guidance of elders, like in Boy Scouts where we boiled a quart of water in five minutes during fire-building competitions. As I grew older, burning wood became a way to heat our home. Wood cutting and splitting became not only a chore but a workout and a way to get my angst out with each strike of the axe.
After I learned metalworking in art school, I started to apply my skills to making wood stoves and thinking about what happens inside of a stove- the mysterious process where tons of wood are reduced to a small amount of ash, carbon dioxide, particulate, and other emissions that are carried away by the wind. The heat is from the atomic bonds of carbon molecules breaking, turning matter back into the energy of the sun that formed those molecules. There is something seductively simple in that balance of carbon in/carbon out, but, as we now know, there is also something deviously complicated about the carbon cycle. I have warmed myself with wood heat over the years in my home, shop, and sauna; each time I light a fire there is still this fascination and allure to the flames that draws me to it and perhaps blinds me to greater issues.
In the past 50 years, wood stoves have gone through several changes. Initially they were simply boxes with a loose-fitting door and a chimney—like the Ben Franklin stove. These stoves burned uncontrollably and inefficiently and needed constant feeding. Later, air-tightness became a thing: dampers were dialed in, rope caulk was added to the doors, and the fires were slowed down so they could burn all night. But as the fire burner longer and cooler, not all the organic matter was burned— more went up the chimney. Flammable wood gasses called creosote condensed on the cool sides of the chimney, building up thick tar-like layers. Eventually that caught on fire, sometimes taking the whole house with it. In the 70’s we went though the energy crises and wood stoves became a very popular way to deal with spiraling oil cost. The 70’s also saw the birth of the environmental movement and the EPA. The EPA stepped in as did Underwriter’s Laboratory (UL). Stoves had to be made safer and cleaner burning. Expensive Catalytic convertors—like on your car—were added to the stove outlet, to capture some of the nasty stuff. But neophytes, in their craze to burn wood, skipped the all-important step of letting firewood dry at least 2 years. The converters clogged up. The cats were dropped, and the focus turned to better engineering. Stoves are now designed with all kinds of baffles to get the wood to burn cleanly; they are complicated affairs, and many don’t work that great- they certainly don’t burn all night, or, as I used to do with mine, burn non-stop, all winter.
But sauna stoves are a different beast. Since they are “occasional use only” (and only one is UL listed) they are, thankfully, technically EPA exempt. But still, I don’t want to be “that guy” that smokes out the neighborhood every time I light my sauna—especially since this past summer when we all got a taste of the Smoke Armageddon. So, despite my years of wood-burning experience, I am always trying to tweak the process and learn the idiosyncrasies of my stove. Every stove fires differently and even a familiar stove can rebel on you when the wind changes or you when move the sauna. When I light mine, I know it will smoke some; my goal is always to get it burning hot as fast as possible so the smoke will be minimal.
The three sides of the fire triangle are heat, fuel, and oxygen. A perfect balance gives a cleaner burn.
You can easily adjust the fuel and air but not so much, the heat. The heat in a wood stove comes from the fire itself, so, you need to get the stove very hot, as quickly as possible, to achieve a good balance. Wood emits gas when is heated above a certain point in a reduced atmosphere; this gas will burn cleaner than the wood itself. If you get your stove so hot that the wood gas burns before the wood, it will burn cleanly. There are cars designed to run on wood gas: a heated tank of wood chips creates the gas that runs the engine. Most stoves have a baffle or two and an upper chamber where the hot gasses will hopefully combust when mixed with additional air; the real heat is at the top of stove, before it exits the flue. In my Lämpimämpi stove, the top plate, that the rocks sit on, is 3/8” steel. I will get this steel glowing a dull cherry red (about 1300°F). Any gasses passing through this chamber will be burned. But until the upper chamber is hot, gasses and particulate will escape up the chimney and the sauna will smoke. Having a brick-lined fire chamber will help the fire get hotter faster. Wet cool days will make it worse as will a down-draft caused by the sauna being in the lee of nearby trees or structures. Wet wood doesn’t help either.
Up until this week my process has been to get a small fire going briskly, with the ash drawer open, and stove door open until it starts to roar (I have an external feed, so no worries about embers falling out.) Then I add larger sticks in one or two loadings until I fill the fire chamber (nothing bigger than my arm–scrap 2×4’s are perfect) topping it with one or two small hardwood logs. The problem is, when I add the fresh wood on top of the fire, there is a period of incomplete combustion as the wood heats up, and the stove smokes a lot. If the wind is wrong, my neighbors will get smoked out. I tinker with the ash drawer or open the stove door to blast in more air until the smoke clears (another advantage of an external feed: I can watch the chimney.) I can add more air to balance the fuel, but I can’t add more heat. Think of it like the carburetor on an old car. Too open, it won’t run well, too closed, it sputters and smokes and clogs the engine. Not enough heat, and it won’t burn well either. I try to find the sweet spot. Unlike a wood stove in a house, I’m not worried about things getting too hot- better than too cool. It’s not uncommon for my stove pipe to glow red for a while—that’s ok, because I know my installation is safe.
Recently, after reading an online post, I tried a new way to fire the stove (yes, old dogs can learn new tricks): from the top down! I load up the stove with larger sticks at the bottom, then smaller, with short sticks crisscrossing between them. Then on top of this stack I put the wads of newspaper with a handful of kindling and light that. The fire immediately starts heating the baffle and upper chamber as the fire slowly works it way down. This way the flames aren’t cooking the larger sticks before they are ready to burn. This solves the too much wood/not enough heat problem. Amazingly it only takes 5-10 minutes for the fire to reach the bottom and a hot bed of coals forms quickly. I leave the ash drawer slightly for twenty minutes and then add more wood. After that you can’t even see any smoke. When we take our sauna to one of the local parks, we can be clandestine; with no tell-tale smoke; passersby have no idea that our sauna is cranking hot inside.
I know there is a whole argument for decreasing our carbon footprint as much as possible and not burning any wood; but there is an opposing argument that says we need to maintain our ties to nature to want to save it. Controlling fire is not only as old as mankind but one of our defining traits.
Without getting into the debate, which I’ll admit I don’t lose sleep over, I will admit that I don’t want to be “that guy”. I want to remain sensitive to others and burn my stove as cleanly as I can. Learning how to master the art of fire building is one small step to take if I am going to cling to tradition and enjoy a really smokin’ hot sauna.
If you look “Podunk” up in the dictionary, it will tell you that it is a hypothetical or insignificant town. The folks who live there think otherwise. Podunk is actually a place name on the map a short ski south of Trumansburg, New York, where I grew up. Despite having only a smattering of residents, they will all tell you that is very real and very significant.
In the 1960’s Ozzie Heila settled there with this family on an old farmstead established by an even older Finn who first built his sauna (above) before the house in the 1930’s. It is also where I learned of all the important things in life. In the 1970’s I spent countless winter hours there at the ski center that Ozzie established, becoming a become a damn good Nordic skier and developing a life-long passion for the sport.
In the summers I explored the creek with his son, my good friend Daniel, and learned the value of immersing one’s self in nature. Daniel’s mother, Ethel, was my art teacher in middle school; she helped me become the artist I am today and we still have wonderful conversations about color theory and art composition. And at the heart of the complex of dated farm buildings was the sauna; there I learned to channel my need to experience extremes into something healthy and life affirming. We loved going from the hot to the cold.
Jumping in the creek in the dead of winter after a searing round in the sauna, we felt more alive than ever. That feeling has never died; each cold plunge I take during sauna takes me back to that creek.
Today, Daniel and his family were back in the area and we went to Podunk to visit the old homestead once again. This time we took our Finnish Blue mobile sauna and parked it next to the ramshackle old sauna, which is now defunct and awaiting a rebirth. Many things have changed but some things are the same. The trees have grown huge or even died, the old purple Lilac, with the rusty sauna bell hanging from its branches, is gone and the brush has been cleared away from the old sauna, revealing the sagging bones of the century-old structure. But the building itself is as recognizable as the last day I took a sauna there about 25 years ago. The inside is a sadder story—it turns out that squirrels like the sauna too and they have made it theirs. As if in a expression of horror at the mess, the Lämpimämpi stove I welded up for Ozzie in the 90’s sits with it’s mouth rusted wide open.
The path through the field to the creek is the same but with a detour to the left towards a new dipping hole: a bathtub in the midst of the rushing current with a strategically placed rock to help keep your butt moored. The run down to the creek had the same awkwardness … trying to run all out before you cooled off but trying to maintain stable footing the same. And the sensation! The whoops and hollers of 12-year-old boys came out of us as we braved the icy April stream.
Real or not, Podunk is the same as it will always be. What are memories but unreal fragments in our minds, ready to be stirred up by whirling waters of a cold stream, or by the hot steam of a sauna? The old next to the new will always appear old, until we make it new again and live our lives in the now, to the fullest, with no regrets, and dreams, not of memories, but of tomorrows.
New sauna and parked it next to the old sauna near the creek.
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.
