In order to save it, the old Sauna at Podunk had to be taken down. The squirrels had taken over and filled the dressing room with a cache of nuts. The building was slowly sinking into the earth and the safety of the chimney, a heavy cast cement affair supported in the ceiling by a rusty homemade contraption, was questionable. The gaping mouth of stove door was rusted open in a permanent state of whoa. If this sauna was ever to make löyly again, work would have to be done. So, a month ago, after careful consideration and much debate, Scarlet and I joined members of the Heila family for a day of deconstruction.
As you may recall from earlier posts, (Sauna Time, Sauna Ritual,Homecoming, Back to Podunk) this is the 90 year-old sauna where many of us locals were initiated in the joys of sauna during the heyday of the 70’s when the Podunk Ski Center was a mecca for Nordic Skiing and all things Finnish. Its simple rustic character, which addressed the basic functionality of the sauna with what I call Finnish pragmatism, is the inspiration behind much of my sauna building. The demolition would give me the chance to dissect it and uncover some the secrets of its original design.
We always thought it was the perfect sauna: hot but airy, it made good löyly, and was roomy enough for an intimate crowd of 8.
What I did not know was how the materials related to its function: how well it heat up, how it held a good Löyly and never felt stuffy, and why it never burned down. Aesthetics aside, these are essential components to a good functioning sauna. We often debated whether it had any insulation at all, so I was especially curious about that.
It was a drizzly morning with a chill to the air; ironically, a perfect day for sauna. Our plan was to document the existing structure and take it down methodically, saving what we could and carting the rest away. Eventually the structure will be rebuilt, as close to the existing as possible, on the same site. We proceeded quickly, each of us attacking an area. Beloved details like the doors and little shelves in the dressing room were labeled, wrapped and carefully stored. The barn board siding was carefully removed board by board, and the whole front facade was Sawzalled off and preserved. As the layers were peeled back, we discovered not only that there had been several incarnations to the structure but we uncovered the answers to some of the questions I had been pondering. There were several surprises.
As the walls were removed from the outside in, we uncovered many layers and each wall was different. On the east wall, under the vertical reclaimed barn boards (installed in the 1970’s?) was a layer of Inselbric, the ubiquitous and horribly ugly asphalt siding that was used starting in the 1930’s. It was easy to use and durable and is still found on many “economy” (or as my Dad would call it: “Early American Poverty”) style homes dating between 1930 and 1960, until aluminum siding became popular. This was over a layer of horizontal 1×6 pine boards, loosely spaced, which went around most of the building. Under this was the big surprise: flattened cardboard boxes, several layers deep, between rough sawn vertical framing members about two feet on center.
The cardboard was in good shape and the labels were easily read: cereal case boxes from Wheaties, Corn Flakes and others. This was the insulation we all wondered about!
A web search of the logo style led to verification of the 1935 date of construction. Interspersed with the cardboard were vertical boards with no apparent purpose. Was this to add thermal mass to the walls? The interior surface was initially all Beaverboard, an early fiber board, which was covered with a thin veneer of plaster (real plaster, not joint compound) which was painted. This was akin to the plaster and tile block sauna of Van Buskirk Gulf I wrote about in a previous blog. This would have provided a vapor resistant barrier that would have held the Löyly steam for the right amount of time. Later, in the 1970’s, this was covered with 1×6 tongue and groove knotty pine. With our current obsession over cedar (or other wood) interior walls wonder if a more authentic sauna might be simply plaster with wooden benches and back rests? The plaster and paint layers (probably lead) were vapor semi-impermeable and thus capable of holding some of the moisture. Surely all the outer layers in the walls were breathable, that is, allowing vapor to easily escape and not collect as condensation, which is a very important consideration in any kind of construction. But I did notice one corner post had signs of severe rot. Did the plaster layer crack here and allow moisture to saturate the wood, setting the stage for a colony of carpenter ants to move in?
I also noticed that, other than the entire building sinking into the earth, the walls were structurally sound. So much so that when Tom hooked up the tractor to pull the north wall off, the whole remaining building (already missing its east and south wall) simple hopped along the foundation slab behind the tractor, taking the chimney with it and sending me into a fit of laughter. All those random layers of heavy boards were keeping things together. It’s not a recommended practice, but sometimes just heaping layers of wood into a structure creates enough redundancy to make it solid. I prefer the more efficient approach of building more with less.
The ceiling was like the walls, with plastered Beaverboard covered by pine. The tiny attic space was filled with a layer of cellulose interspersed with rodent droppings, walnuts, empty boxes of rat poison and a few old bottles, which probably once contained hooch. One was verified as being from 1938 by its unique design. Probably teenagers hiding their stash after a sauna; but, quite possibly, offerings to the sauna Gods to protect it from burning down.
As for fire safety, it was a miracle that the sauna never did burn down. There was a lot of charred wood throughout the attic, especially around the iron chimney supports.
Again, there were a lot of heavy boards, which seemed to have no structural significance, perhaps only adding thermal mass or insulation. The roof rafters were so heavy and the roof so strong that after it was lying on the ground like a low pup-tent, Tom had to drive the tractor over it to break it apart. The metal standing seam roof, with its many coats of black tar, was in surprisingly good shape, but leakage was occurring where the heavy, cast refractory cement chimney penetrated it. The stove below, welded by me in the 1990’s, was so rusted it was deemed to be scrap.
The cement floor had sloped to a drain but was cracked and broken. The original cement pour seemed hodgepodge and lacked any re-bar. Woodchucks had tunneled voids underneath it. The drain allowed for bathing— something the early Finnish farmers needed as the house probably lacked plumbing; bathing, to me, is an essential part of the sauna experience; that function of the sauna informs my designs. The floor will be replaced with an edge- thickened slab as the foundation; with a solid gravel base over undisturbed earth and with steel reinforcement.
The one component that perhaps was a factor in why the sauna felt so good was all the brick work around the stove, which was fired through the dressing room wall—a traditional design I frequently use. This added about a thousand pounds of thermal mass around the stove. Thermal mass holds the heat and radiates it back into the room but also means it will take longer to heat up. I typically use a lightweight fire wall so the sauna will heat quickly and to lessen the load on the building structure, but perhaps I should re-think that and revert to the solid masonry I started building with in the ‘90s. Ironically, the brick work at Podunk was added in the 70’s. The old Finns around here commonly relied on asbestos board for fire protection.
By the end of the day, we had a pile of barn boards and other parts stacked and labeled in the old ski lodge, and a dumpster overflowed with the rest. Although most of the sauna was discarded, the lessons learned will live on in the saunas I continue to build. Next year, we will rebuild Podunk with modern efficiency but in the same basic footprint as the original. Hopefully the entire facade will be replaced and the lilac tree where the sauna bell hung replanted. We’ll probably skip the lead paint and asbestos board and use a modern, UL listed chimney support in lieu of the home-made rig that was there. Fire safety will be based on science, not luck. Cedar over foil (with an air gap!) will line the walls and the functionality will be the same, and hopefully, better.
Family and friends will gather there to sweat and bathe and run naked to the creek for generations to come.
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.
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.
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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The Kiuas, or heater, is the heart of the sauna. In a wood-burner, it is commonly referred to as the sauna stove, but a wood stove it is not! There is a lot of misconception around the kiuas and how it is different from a wood stove that you might use in your house.
First, some history. The modern house stove is really a heating device designed to add comfort to your home while conforming to certain safety and smoke emission rules. Typically they are not used as primary heating appliances, unless you live in a cabin off-grid somewhere. Back in 70’s, during the energy crisis, woodstoves become popular as a way to save money. They were pretty much unregulated and varied in design from a kit that consisted of a door and a flue collar you could slap onto a used fifty-gallon drum, to a more complex Vermont Castings wood stove. Earlier stoves had little control over combustion; these evolved into airtight units that could keep a smoldering fire all night, if not for an entire season. I had one of these and didn’t let the fire go out all winter except to clean it. Cleaning the chimneys on these was imperative: when wood—especially if it is not cured for two years— is burned slowly by reducing the combustion air to near nil, creosote forms as the result of the resinous gasses condensing on inside of the cool chimney walls. As a result of the slow burning, these stoves emitted a lot of smoke. After many houses were lost to chimney fires, safety regulations were put into place and stoves were required to use catalytic converters, much like on your car, that reduced emissions. These required a religious adherence to the use of dry wood, lest your catalytic converter clogged up, which most of them did. Those evolved into today’s stoves that use a carefully designed system of baffles and airflow to make stoves burn efficiently. Now, all wood burning home heating device installed in the US must comply with UL (Underwriters Laboratory) safety standards and increasingly stringent EPA standards for particulate emissions. The stoves work well and are very cozy but, by design, they don’t get very hot fast, nor are they meant to burn all night long, not to mention all season long. Because they are intricate with interconnected parts, they are all cast iron. The exception is some stoves made in the pre- catalytic converter era, which were welded steel.
So, that is a wood stove. You may find a used one and think you can build a sauna around it, but the truth is, with the rare exception of one of those 70’s all welded steel (but not a barrel!) stoves, you can’t. You can build a small hot room with a wood stove, but it will never be a real sauna; here is why: A sauna stove, or kiuas, is designed to do one thing: heat sauna rocks. It is the hot rocks that heat the sauna, that produce the burst of löyly steam and that are true heart of the sauna. Early saunas did not have a metal stove- they did not even have the technology to make a metal stove- all they had was wood, earth and rocks. The kiuas was essentially a hollowed out a pile of rocks. The fire was lit within, the room filled with smoke, and, after the rocks got hot, the fire was extinguished and the room cleared of smoke and then the rocks heated the room. The closer you can get to that smoke or Savusauna experience, the better.
A sauna stove is not a wood stove; it fires hot and fast, it burns sticks, not logs. It’s job is to heat rocks. If fired correctly, you will never have to clean the chimney- a hot fire will combust all of the sticky wood gas. It is welded steel—which can have water poured over it while red-hot. Cast iron cracks or explodes when subjected to this. It can take the weight of a hundred or more pounds of rocks sitting on top of it when cherry red. Ferrous metal takes on specific color when heated; at 1400° F. it is cherry red. At that temperature, an 1/8 inch plate of steel is as malleable as taffy on a hot summer day at the beach. I repaired many sauna stoves where the top looked like an egg carton from the stones pressing down on the hot metal, so I started making stoves with half-inch thick plate at the top. I fire my stove so hot that I see dark cherry red glowing underneath the stones. If you fired your home heating wood stove like that, you would be crazy. I swear that sometimes I could read a book by the glow coming off of my sauna stove. I like to push the limits so that I know it is safe.
When you light a sauna stove, you want to fire it, that is, to bring it up to temperature quickly. Use paper and dry kindling and then stuff it full of sticks, not logs (wood scraps from building saunas work great). Because they are for intermittent use, they are exempt from the EPA particulate rules but, the truth is, once it gets going—after about ten minutes—it should burn so hot that there is no smoke at all. Other than the shimmering light from the heat, I can’t tell my sauna is heating up by looking at the chimney. House woodstoves are tame devices, meant to be safe. Sauna heaters are another beast; that is why I will never install a wood burning kiuas in a sauna in or attached to a house. They do burn down now and then.
If you are building your own wood-burning sauna, you may have a building inspector involved or have to get a wood burning appliance inspection for your home insurance and that may require a UL listing. The only heater with UL label on it is the Lamppa Kuuma stove; most of the others are made for the European or Canadian market, which uses different standards, so before you click on “buy” you should have a conversation with any inspectors involved. They may love the idea of a sauna or they may think you are crazy to sit in a small hot room and throw water on a red- hot wood stove, in which case you’ll have to convince them that it’s something that’s been done millions of times without incident. In any case you will need to make a safe installation of your Kiuas – there are clearances and heat shields and floor hearths; none of these can be cheated on, unless you don’t mind owning one of the ones that burns down. There is also combustion air to consider- which is why I like to fire mine from the outside. The sauna stove sucks up fuel and oxygen, better to not be sucking the air out of the tiny room you and your friends will be in. Not such a problem with a house wood stove (but with newer air tight construction and tiny homes, it is).
So, before you get that old woodstove you find on Craigslist, do your research and think about investing in a real sauna stove because the kiuas is the heart of the sauna and the kiuas is not a wood stove!
My stove pipe will get cherry red!
After ten minutes, a hot fire fire should burn clean and smoke-less
All stoves will smoke initially
Nothing bigger than arm your arm!
This armload of ash (the wood) will fire the sauna
Fill it and let it burn hot before closing the ash drawer
