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
My latest mobile sauna project started when a customer contacted me wanting a mobile sauna she could use for hot yoga. It’s my standard five by eight foot trailer sauna but without the benches. The arched roof allows for plenty of room for stretching out and the floor is soft cedar. The heat comes from a Scandia propane-fired heater. It’s a cute little unit that weighs under 2000 pounds. Tow it to the next yoga retreat and show off your downward facing dog
When the client for my latest mobile sauna project contacted me, he told me he needed something that would look and feel like a sauna from back home in Finland. He wanted it to be wood-fired and to get really hot. He wanted the clean lines and rustic charm of Finnish design and even requested a traditional pine tar finish like what the Vikings used on their boats. As small as it was to be, it was to have the standard two rooms- the sauna room and a dressing room. He also wanted to use the latest solar technology to light it with a soft glow.
But, working for an American company, where he might get moved from time to time, he wanted it to be un-tethered to his house, to be portable so he could always bring it with him, like a cherished possession.
I enjoy challenges–in fact, I thrive on them. One of the advantages of having my own company is that I get to decide how much to put into each project and which projects to really focus on. On some projects, like this one, I get to expand my repertoire. The goal, as always, was to bring my client’s dreams into reality. The result: a mobile sauna on a 81 by 120 inch trailer, under 3000 pounds, with two rooms, solar powered lighting, custom wood stove, northern white cedar interior, and pine tar exterior finish, did just that. I created a little oasis— a reminder of Finland—to park in his back yard, a dream come true.
Saunas are like that. When you have your own, it is a dream come true, a special place to escape into, to relax and unwind. It is tied to old traditions but for many, it is a new experience and can be life-changing. As designer and builder I get to be the midwife for people’s dreams and help them usher in a new way of living or rekindle a past love. As we turn the page to a new year and think about resolutions, what dreams do you want to come true?
mobile sauna by rob licht Custom SaunasSolar powered lights on mobile sauna by Rob Licht Custom Saunas
I get a lot of questions regarding sauna insulating details and thought I’d shed some light on a few issues. A caveat before I start: heat transfer science gets pretty complicated and I am grossly simplifying things here. I’m not an engineer but I rely on experience and am constantly probing and measuring my own saunas to see what works. A building inspector may want an engineer’s input, but just make sure the engineer understands what happens in a sauna.
If you are building an electric sauna, either in your house or as a stand-alone building, you’ll naturally want to insulate it for efficiency. Normally builders (and building inspectors) think of R-value (printed on every insulation product label) as the golden metric, and the R- values of a wall assembly are typically added up to get a number that either complies with code or satisfies a self imposed trade-off between cost, efficiency and practicality. R Value is the resistance to heat transfer but measures conduction and convection, not radiation, which is not much of a factor at lower temperature differentials. R values are calculated with normal living spaces and long term heat retention in mind, which in a typical home is calculated using an average temperature differential of 24°C (between heated and outside space). Since R= Delta T/ QA , (where QA is the ability of the material to transfer heat) and in a hot sauna Delta T might be 100°C, the use of labeled R factors is totally skewed!
The second factor is time. Heat loss is measured in BTU/ hr. With the sauna only on for few hours a week (bravo if it’s more!) your heat loss will be minimal and hopefully, in the cold months, will contribute to heating the house. So, in terms of cost vs. efficiency, a lot of insulation may be over kill.
At the higher temps of the sauna, the radiant effect of heat is more of a factor and the use of a radiant foil barrier comes into play. The heat you feel radiating from a wood stove is the long wave radiation. This radiation can move through common building materials but foil stops it dead in it’s tracks. Anyone who has nestled under an emergency blanket or protected himself from the fiery of a blast furnace, like when I pour bronze, understands the effectiveness of foil to bounce radiation back towards the heat source. But if the heat source contacts the foil layer, the aluminum superbly conducts the heat, defeating the purpose. So, when building a sauna, it is the radiant foil layer, with an air gap (on the hot side) that is crucial to holding the heat in. This should be backed by as much standard insulation as is practical, but don’t worry about attaining super R – value. The exception being if the wall is an outside wall of the house and a part of the building envelope- then, R-value must be a minimum of what the rest of the house has. I prefer Mineral wool, but in any case do not use XPS or EPS foam directly behind the foil, as they will melt at sauna temps!
Vapor control in an interior sauna is really important especially in modern tight houses, which tend to trap moisture, both for the damage vapor can cause that you can see, such as peeling paint, but more for the damage you won’t see, like moisture condensing in a wall cavity. Radiant foil barrier, when carefully taped at the seams, is also a perfect vapor barrier. When I build interior saunas I think about all of that moisture and imagine where it can get to and wreak havoc. I then seal off those spaces but provide a vented path for it to escape. Some enthusiastic löyly action will turn ladles of water into steam which fills the sauna but then escapes into the house— like when you forget a kettle on the stove and all your windows fog up. The best thing is to build your sauna next to a shower area and then vent that area with a decent bath fan to the outside or via the household HRV system. The sauna should then have an air intake under the heater as per manufacturer’s instructions and via a gap under the door so sauna gets a healthy exchange of fresh air. Never connect the sauna directly to a mechanical ventilation system.
With careful planning of layout, insulation, ventilation moisture control, and a heater that makes good löyly, your indoor electric sauna can feel like a wood burner on a pond’s edge but also be an integral part of your efficient home. (Read more about Sauna foam and best sauna building materials at Sauna Insulation, Revisited)
Sauna is all about perfection. Not over-the-top polished perfection, but a perfect way of being: simple, pure, functional; perfect living. Harmonious. After all, you enter the sauna naked, our perfectly imperfect bodies exposed but hidden in the dim light. You sweat out the toxins of life and leave with a clean aura. Like the Japanese concept of Wabi-Sabi, the sauna encourages acceptance of the imperfect as natural and beautiful.
When I work on my saunas I am constantly aware of this. Too much perfection will ruin the relaxed atmosphere; too many crisp details will hold tension in the materials. I relax when I work, become one with my materials and try to imbue the building with a human inexactness.
It’s all made by hand or by nature: the pulls on the doors (hickory branches), the handle on the stove (me, wrapping stainless rod—like wrestling a snake); the benches (massaged with sand paper) and the funny round window (imperfectly round like the eye of a whale). The stone facing on the wall around the stove was pulled from a hundred-and-fifty year old barn foundation and carefully split with whacks from the hammer my great grandfather used to carve head-stones. The dressing room floor: reclaimed fir, every bit as tough as the day the trees were felled. I use some new materials but never looking like I just pulled them off the shelf in some big box store.
I’ve touched it all many times—each board, each stone, each piece of metal. I carry slivers of each project in my hands for weeks—a constant reminder of the work I do. I think of our physical world built by hands. Every brick in every building handled, touched and in the memory of some callus; everything we think of as solid and real created by someone’s toil. Even the rocks that mark the hedgerow at the back of the sauna were placed by hand almost two centuries ago; the sweat of that farmer’s labor infused with this thick clay soil.
This last project was nearly perfect—which is as close to perfect as I want it to be. Great client, perfect site, easy access, and nice new pond with a beautiful dock and deck. Ok, I did order the wrong color roof but the multiple drives back and forth to Mid Lakes Metal, down the spine of the ridge between the lakes was perfect. With my windows wide open, I could taste the salt of the earth and was reminded of why I call this place home.
All summer long I have eagerly anticipated this week; we have a cottage rental on the lake. It’s the highlight of my summer and a much-needed break from all of the projects I have going on. This year, in addition to the usual activities—swimming, canoeing, beach fires, collecting beach glass and just staring the waves while sipping wine—I’ve added one more: sauna! I’ve brought my wood-fired trailer sauna with me and parked it ten feet from the water’s edge. Nothing beats coming out from the hot steam of a good löyly and jumping into the cool, refreshing lake. It is perfection.
Mobile Sauna by Rob Licht Custom Saunas on Cayuga Lake
