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!
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NOTES:
- Robert Siegel and John R. Howell. Thermal Radiation Heat Transfer; Volume 1, 4th ed. (Taylor & Francis, 2002), 7. ISBN 978-1-56032-839-1 “A blackbody or black body allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body).”. For more information: https://en.wikipedia.org/wiki/Black_body ↩︎
