The Science of Sauna
The one thing that always comes up when people ask me questions about building saunas is: how do you insulate it? Intuitively, one might think that the sauna, with it’s high temperatures, would need more insulation than a house and should be as tight as possible to conserve energy. In fact, I’ve had building inspectors give me a confused list of requirements using such logic. The reality is that a sauna is such a different beast than a living space that most of the calculations have to be thrown out the window. R-value, the number printed on most insulation products, is the resistance to heat flow of a given material for a given thickness for a given temperature difference (delta T) between the hot and cold side of the material. Typically, in our region, delta T is assumed to be 35° but, in a sauna, the delta T might be 165 degrees Fahrenheit! So, in terms of heat loss, we get some very different calculations! To really understand R-value, you need to think in terms of it’s inverse: the U value, or coefficient of heat transmission. U value is expressed in units of Btu/hr/sq. ft./°F, or, plainly, how much heat is lost per square foot for every 1° temperature difference. A typical sauna, with R-13 average insulation, might lose 4000 Btus per hour (or 1200 watts). But, A typical sauna stove generates 25-40,000 btus of heat per hour, so losing 4000 btu’s is not a big deal. (It is more important if you use an electric heater: an 8 kw unit, can only put off about 20,000 btus in an hour.) The other factor to consider is that, unlike a living space, you are not trying to hold the heat for very long. So, you don’t need to stack up piles of insulation in the walls and ceiling. In fact, many old saunas had no insulation at all.
What R factor does not measure though, is radiant heat flow. Radiant heat is like the sun warming your face; it is the short wave radiation that you feel. At higher temperatures, short wave radiation becomes a bigger factor than convection. To contain that radiant heat, we use foil, (think: thermos bottle or emergency blanket) but foil, being highly conductive, only works if there is an air space between it and the heat source. The foil doesn’t have to be visible to work; it can be buried between other layers of materials. In my Saunas, it is behind the cedar, with an air gap.
Saunas are also not meant to be tight, stuffy boxes. They require airflow to move the heated air and steam and to make them comfortable. The old Sauna at Podunk was the best one around because it was old and drafty and it always smelled fresh. Counter to today’s high tech homes, ventilation has to be has designed into the sauna room to let it breath passively; the trick is to do it without creating annoying drafts.
When you sit on the bench and enjoy the relaxing warmth of the sauna, you probably aren’t running all of these calculations through your head- and neither am I! What I do know, from 40 years of sauna experience, is what does and doesn’t work. Mostly, you want a good pile of rocks (kiuaskivet) that are hot enough to alternately bask you in their radiant heat and make good steam (löyly) and convective heat off of the heater (kiuas) to produce waves of heat that gently wash over you as you breathe in the fresh aroma of the sauna. You want air, some light and a feeling of openness and connection to the outdoors. It’s not so much science, as it is art, or perhaps a melding of the two.
If you do have specific questions about designing your own sauna, feel free to give me a call or email. Or better, have me come over for a consult. If you live far away, I can even do one-hour phone or Skype consults. In the future, look for classes I’ll be offering on Sauna building, where I can go over all the science—and art—of Saunas in greater detail.
pouring water on the rocks 
Recently, I was called upon by a couple who had bought a property with an existing sauna that needed some professional help. The structure was a cordwood affair: a building method where timber framed walls are in-filled with short logs, stacked like firewood, with mortar between them. It looked like it came right out of the pages of Rob Roy’s classic how-to book: The Sauna. The exterior was fairly solid and quaint in a hobbit kind of way, but the interior, in terms of functionality, had some serious flaws. The timber frame and log and mortar walls looked pretty good, but it is obvious that the project was a classic DIY affair: a case of a homeowner taking on a project that looks easy, but with complex details that get skipped due to a lack of knowledge, funds or basic skill.
