Candle windows hark back to my time at Podunk, where the light in the sauna came from a bare bulb in a porcelain fixture outside a little square window into the dressing room. A sauna is too hot for a standard light fixture, so this arrangement made sense. Later, after I started building saunas, I learned that this was a more modern incarnation of the original candle window, which was literally a window into the dressing room with a shelf for a candle to sit on. These windows are common in Finland in freestanding saunas away from the house. The candle window allows a special, spiritual, summoning light into grace the sauna. Especially on those dark winter nights.
In the sauna tradition, we slow down. The flickering candle-light, seen from the bench in the sauna, lures you to relaxation and reflection. Life and relativity. Could there be a more tranquil way to release the stresses of the day?
Although it is this quality of the light that is so important, the candle window is totally pragmatic in a very Finnish way. A candle in the sauna room would melt even if not lit, so this setup was an obvious solution to the problem of lighting the dark interior of the hot room. Despite its pragmatic origins, I find it is also a chance for a little expressive design: it can be round or square, arched or colored. It can have an organic flare to it. Now, with cheap, battery-operated, multi-colored LED lights and even fake candles that look real, the light can be more than a simple bulb on a pull-chain porcelain fixture and be safe. Even if the sauna has built-in electric lighting, the candle window can be a signature element, one that distinguishes a personalized custom sauna from a generic kit.
It’s in the details.
Finnish pragmatic design inspiration comes from using what is available at hand and letting that material influence your design. There are many places to incorporate little details and personal touches: stick hardware towel pegs, stone-faced walls with stones from your backyard, thresholds of locally cut locust, round windows, etc. Think of decorative elements you can hang above the mantle. In my sauna building plans (which can be purchased and download), there is more about windows: framing information and tips on using windows safely in mobile saunas.
Wood-burning sauna with candle window to dressing room.
Here is a collection of the candle window designs and builds I’ve used over the years in and around the Finger Lakes and throughout New York State.
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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