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.
Building a sauna requires many skills. Basically it is a small house; there are windows and doors, a roof and a foundation; framing, sheathing, subfloor, and the like. It also requires a design, and many cases, a permit, which will include drawings such as a site plan, showing required set backs and orientation. All of this I can do— from plans and permit applications to foundation to chimney. I pride my self on being able to do it all and to being as comfortable holding a drafting pencil ( yes, I do drawings old-school) as I am a pick-axe or nail-gun. But the truth is, sometimes it is best to let others do the work they do best so I can focus on what I do best, which is the sauna.
Recently I had a job where distance made it much more efficient for the owners to use a local contractor to build the shell while I did only the sauna interior and the overall design. It turned out that Tim and his crew were much more adept than me at not only building the shell but carefully replicating the trim details of the 150 year-old adjacent main house. By the time I got to the job site the interior was ready for my sauna work.
Just like everyone else during the pandemic, planning sessions happened on the web or via text; we only actually all met once. Despite that, or maybe because we weren’t in each other’s hair (a sauna is, after all, a very small space), things flowed very smoothly once we got over the scheduling speed bump caused by the pandemic-induced supply chain upheaval. The sauna sits perfectly between the historical architecture of the house and the modern look of a contemporary sauna. It was a team effort that paid off.
When building a sauna the first and possibly most important consideration is the location. With a wood-burning sauna, which is free from the tether of an electrical connection, it can be away from the house—not just for safety but also to create a separation from the electrical buzz of modern life. Simple and inexpensive solar options make it easy to provide needed lighting. It should have some of the comfort of home but be integrated into nature; near a body of water is always a good choice.
My latest sauna does all of that and more. The site is fairly close to the house but lies outside the garden gates. It all but hangs on the edge of a small gorge that contains a lively creek in its serpentine confines. Like the basswood and cherry trees that cling to the sides of the gorge, firmly rooted in the ground, the sauna is anchored to the 300 million year old shale bedrock with concrete and steel. The owners built a steep stair down to where small waterfall flows into a perfect bathtub sized hole. Descending it may be slightly perilous but that only adds to the adrenaline to rushing from the steaming of the sauna and plunging in the ever-cold water.
As I build I tweak my design to allow nature into the sauna. Framed and without sheathing I could see the perfect view up the ravine from the upper bench, suggesting the optimum location for a small candle window. The large window allows a view of the wooded hill and brings in ample afternoon light and the view down into the creek through the framing allowed me to imagine the possibility of a small square creek view porthole below knee level that would let in the ambient sound of the rushing creek. Exiting the sauna one faces the woods, not the house; a crude stair-path leads the eye up into the forest while the other leads to the creek.
The sauna is visible from the road and the house, but neither is evident from the sanctuary within. All you hear is the babble of the creek and all you feel is the relaxing heat of the sauna. Finishing a few rounds in the sauna with a dunk in the massaging water is pure bliss.
The site not only perfect for the sauna but it was a joy to work there, listening to and watching the water flow. Daily I took dips in the creek to beat the steamy summer heat. Having a site that allows me to enjoy the process of building lends means I can build a better sauna—one that is infused with the spirit of the place and connected to nature.
Increasingly I hear from clients who want their sauna as a way to enhance their hot yoga (Bikram) practice. It’s a perfect pairing: what better way to follow up (or warm up for) a yoga session than with an even hotter sauna!
Recently a couple asked me to convert an old dingy freestanding cinder block garage into a sauna/ hot yoga studio. First I made sure the cinderblock wall was stable and did some minimal repairs. Then I isolated the block wall from the warm, humid space by adhering expanded polystyrene (XPS) foam board to the walls. This is critical as it prevents moist air from hitting the cold cinder blocks and condensing. Then I framed in the space, insulated the walls with mineral wool batts and finished the yoga space with drywall and the sauna with cedar (with the requisite radiant sauna foil layer and air gap). New windows replaced the old; the dramatic deep window recesses a result of the thick walls. Bamboo flooring over floating sleepers over foam board created just the right bounce for the yoga space, while the sauna has traditional duck boards. LED lighting added just the right ambiance. The heart of the sauna is the Harvia Cilindro heater with it’s 200 pound rock capacity. Amazingly, the old building was plumb and square— the original masons did a good job. Fighting an out-of-square space is the bane of all renovators.
This project was a complete transformation for this building (see below), turning it from a creaky old, under-utilized garage into a revitalized space for self-transformation. Make an inventory of the neglected spaces on your property awaiting transformation and give me a call!
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.
