Lately I have been thinking about the application of the foil I use in my saunas as a radiant vapor barrier. Perhaps this is because it almost Christmas and I was thinking of how we decorated the tree each year. The final touch would be to drape foil tinsel over everything; our mother would have to constantly damp down our enthusiasm by reminding us to place it carefully on each branch, not to throw it.
This suspicious “sauna” foil is Aluminum-coated Plastic—upper working temperature of only 55-120° C.
There are tricks to using the foil but the first and most important step is to buy the right stuff. Like the tinsel we put on the tree, the foil may actually be aluminum-coated plastic— which you don’t want to use. That plastic is likely polyethylene which, if you look it up on the material specification sheet that every product has, it has an upper working temperature of 55-120° C, meaning it will likely melt at typical sauna temps. Sauna Foil, available from any of the familiar sauna suppliers, is aluminum foil on a kraft paper backing. I used to find it with fiberglass reinforcing thread, which is helpful because the stuff tears easily. Also helpful is 4 ft. rolls, rather than 3 ft so you can do a wall in 2 passes, but I have trouble finding this too. I recently tried a new supplier selling 4 ft rolls of “sauna” foil, but upon opening it had a suspicious plastic look to it. That night I put it in the sauna and within seconds it began to distort and curl up like the polyethylene I suspected it was made of. (See illustration above)
The second thing is to design the wall correctly. I read and see a lot of misinformation that touts using no air gap with foil. The air gap is essential. The foil works by reflecting radiant heat. All “black bodies” give off and absorb radiant heat that travels in a straight line from one hotter object to another cooler one, the hotter the body, the more heat it emits. The sauna rocks radiate a “soft” heat to you, the walls, and the benches, and that is why you want the sauna to be laid out so that everyone has a view of the rocks. The fire, if seen through a clear glass door, also radiates heat— but at a higher intensity. Too high for a comfortable sauna (but great for ambiance.) When that heat hits foil, it is reflected back into the room or the backside of the cedar—if there is an air gap of at least 1/2″. If it touches the backside of the cedar the foil— also a perfect conductor—pulls the heat away from the cedar and transfers to the wall space behind.
Air Gap. A Sauna Building Best Practice.
I’ve understood this for along time. The first semester of college I took a class: Solar Design and the Energy Efficient Home. We learned all about insulation, heat transfer and basic building skills. The first day of lab, where we were building a timber frame house, I was handed a Makita 12″ circular saw. My building career started right then and there.
With the web of misinformation out there I had to think of a way to illustrate this basic principle of thermodynamics that I learned my freshman year. So, one slow day in the shop I rigged up an experiment and photographed it. (see illustration below) I set up a section of cedar wall about 18″ from my infrared shop heater and fastened 2 pieces of foil to the back, one with a 3/4″ air gap, and one with no gap. After an hour the cedar was 250°F on the front—like it is often is in my sauna. The back of the cedar was 121° F, which is impressive by itself. The back of the foil with no gap was 115°F, meaning it was acting as a perfect conductor, and the back of the foil with an air gap was 71°F: room temp. The air gap was clearly making a difference, 45° in this case.
The thermodynamic experiment begins.
After an hour on cedar
back of cedar
back of foil, no gap
air gap makes big difference!
The foil is a perfect vapor barrier rated at zero perms— meaning no vapor moves through it. But unless you layer it properly, with insulation behind it, the moisture will condense on it, or the first cold surface it hits. Even in a perfect build, there might be cold spots in the insulation (typically about the size of a mouse hole), so there likely be some condensation, but not a problem if there is air movement. The air gap behind the cedar allows air to circulate around the cedar, removing any moisture and ensuring that the wood heats and dries evenly and remains stable. Heating one side of a board and wetting or cooling the other is how you make curved boat staves.
There are other tricks to using the foil- like unrolling it and re-rolling it foil-in, or using temporary magnets when working a commercial job with metal studs, but the key is to use care. Use plenty of hi-temp foil tape and patch tears as you go and work with a partner if possible.
I suppose you could build a sauna by putting a heater in a refrigerator box- but that would last about a day and be incredibly wasteful. Cedar touching foil won’t ruin your sauna and neither will plastic melting in the walls where you don’t see it. But if you are going to take the time and bear the expense of building a sauna, you might as well do it right and so it will last generations. I guess my mother was right: applying foil carefully and not just throwing it up is the way to work.
You’ve probably heard that I’ve spent a lot of time in the sauna but another hot spot I’ve spent a lot of time around is kilns. Specifically foundry kilns and ceramic kilns. Unsurprisingly there is a strong relationship between the two as they both involve getting things hot. In the lost wax casting process, investment or ceramic shell molds are heated to roughly 1500° F, which burns off the wax original- thus the “lost wax” of lost wax casting. This can take hours or even days depending on the mold type and size. A ceramic kiln can get much hotter- up to 3000° F. That is hot enough to melt steel and many other metals.
I learned how to do bronze casting in Art School. It is an ancient process and we did it pretty much the same way that it was done thousands of year ago. We learned to figure how hot things were by using our senses. All objects emit radiation when heated but at about 1100-1300° it become visible. Peering into a hot kiln (safety glasses strongly suggested) is like looking at another world, perhaps on some alien gaseous planet. Solid objects look like they are transparent. Heat and light become the same thing, the heated molds don’t reflect light but emit light. The blast of heat through the spy-hole is like a ray gun. We rarely used pyrometers (hi temp thermometers) and when we did it was only to affirm what our senses were telling us. We would record the smells of things burning off. When the smells were gone, the molds were clean and ready to accept the molten bronze.
When loading the kiln there is always discussion about the hot spots- certain delicate molds need to avoid the heat while larger molds might need it more. There is always conjecture about how the heat circulates; a whole aspect of kiln building is dedicated to controlling the flow of heat within the kiln. Some of this conjecture is borne out in the results of a firing—whether things fire correctly or not. Ceramicist use cones: small tapering forms that bend at specific temperatures. After a firing these will give a true telling of how the firing went. But, despite the science, there is still a lot of mystery and art to the process, so much so that a firing of a large kiln can take on a ritualistic feeling. Staying up late to tend the kiln, as is done with wood fired and other non-automated kilns, drinking beer and heating up pizza on the kiln, tends to add to the aura.
Thinking of all of this makes me think of sauna. Both have been done pretty the same way for millennia with an aura of ritual and involving community. Both have a focus on fire and heat, and, as well studied and commonly practiced as they both are, there is still a bit of mystery involved in each.
A kiln is like a sauna on steroids. The heat is so amplified that its flow and effects are unmistakable. Observing one is a lesson in thermodynamics. In the sauna building culture there is a lot of banter about how to best heat, insulate, and vent a sauna, yet all of it is conjecture based on theory until one sits in a sauna and feels the heat radiating off of the rocks and the wave of löyly hitting you on the sensitive tops of your ears.
When I design a sauna I draw from my years of kiln experience; I think of the heat as visceral substance, almost visible, as in a kiln. I relish the use of my senses to discern quality rather than depending on technology. Even if the sauna is electric with a digital control panel I rely on feel, not the number on the display. I imagine the flow of heat like the way it flows in a kiln. My foundry experience has informed my understanding of sauna in ways that are hard to describe but suffice it say that I have always been drawn to fire and to the mysteries that it holds.
A lot of building science is pretty theoretical because, 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, but 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 right 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 were inexplicably filling 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 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. (Read previous post about Insulating Saunas)
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 in 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 still shiny! There was some high-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 my disdain of fiberglass type materials (because rodents love it).
polyisocyanurate board, original state
fiberfax mouse nest
Melted EXP (expanded polysytrene) foam
On my desk I have a piece of EXP (expanded polysytrene) foam 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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