During the Pandemic, access to public gathering spots was denied and people rearranged their lives around home. Many worked at home and most, no doubt, played at home and socialized at home. When the dust settled, people saw advantages to this reclaimed space in their lives. Home improvement contractors and suppliers, including me, were big winners in this collective adjustment. My phone has not stopped ringing since 2019.
The home gym market has become a strong area of growth for sauna culture. Even now, when most public gyms are back in full swing after a few years of cautious access (including restricted sauna use), people still approach public gyms with trepidation. Tight, airless locker rooms, equipment shared by hundreds of people, and saunas with a little too much patina all went from being tolerable to a complete turn-off. A shoulder injury while riding my road bike has sidelined my stationary pedaling to recliner bikes at my local Y. Sitting in a room full of people breathing heavily, in isolated headphone bubbles, is not what I call exercise bliss. The facility’s sauna can’t begin to approach the rich experience I write about in these posts. I avoid it completely.
So, it is no surprise that those with the means are investing in their homes and building elaborate gyms with saunas and even pools, where exercise and sauna can be a blissful experience. Where one knows they are breathing only their own air in a space that has been cleaned to their standards and where they can sit meditatively in the sauna and not feel the whoosh, whoosh of the treadmill on the floor above. The blue light screens of illicit cell phones are gone, and conversations with random strangers can’t stray into political abrasiveness.
After the pandemic gave people pause and the opportunity to reflect upon what is essential in this life, it is no wonder that I have been so busy helping people realize this vision of a home sauna. Sauna is essential just as is control over your own health and fitness. Creating the personal environment for it all to come together is a luxury beyond financial means; it is a luxury of thought and intention, of knowing what to value first. If you want to have a quality of life where being healthy and fit means taking advantage of the full potential of your body, then make it a priority to have a space at home where you can exercise and practice mindfulness. A yoga mat in a corner and a few weights or resistance bands is all you need to get started; adding a sauna is icing on the cake.
Recent client’s priorities of fitness, health, and family led them to me. I built a roomy sauna in their home gym, which is one to die for. But what mattered most was not the nice setup or expensive equipment, it was that they had made being healthy together a priority—and that is something we can all afford. Even though I would love to have their gym setup when I am cranking it out, sweating buckets on my old bike-on-a-trainer next to the water heater in our basement, I am just as happy and alive as I would be in their gym (and my wood-fired sauna out back rocks!). As we age, there is no going back to reset the clock; we must simply keep moving and use the body we have. Movement and exercise can be done with little or no equipment, and having a Sauna is a nice reward and perfect compliment. Your body will thank you!
Growing up in this land between Seneca and Cayuga Lakes, I spent my share of summer days loafing about the shores, swimming, skipping the water-worn shale rocks, or just sitting and enjoying the view. Many friends had lakeside cottages, and I always dreamed of my family having our own. Now, with a property in Scarlet’s family (passed down from her grandmother Dede), that dream is a reality. An authentic cottage, in my book, is more than just the simple, rustic building by the water, it is a multi-generational gathering place steeped in family traditions. At times it may be used by a single-family group or even a solo practitioner, but at any other time it may become a gathering place for a large group of cousins, aunts, and uncles, hopefully announced but sometimes not. Sleeping situations may require creative solutions: every couch typically opens out into a bed; hammocks are strung between trees or porch posts; and daybeds are standard for every room. A one- or two-bedroom cottage can sleep ten or more in a pinch. Life is communal and privacy is found on solo swims or paddles toward the middle of the lake. The minimal privacy is not a detriment, but an expected condition taken for granted. This situation may not be much fun for an adult who needs undisturbed sleep, but heaven to a pre-teen who can stay up late and giggle under the covers with cousins for hours.
There is a code of maintenance with cottages. Upkeep is essential as with all dwellings, but here, the maintenance must be almost invisible so that the status quo is also maintained. The human efforts to push back against the forces of nature, which constantly threaten to erode our existence, must be kept in a delicate balance with nature’s chaos. On the shores of a lake, those forces can rear up unpredictably—like when the boathouse roof was sheared off during a storm this spring at Scarlet’s family cottage. But the new roof looks like it has been there for fifty years. The weathervane, with its lake trout constantly swimming into the wind, was rescued from a neighbor’s beach, untangled from water weeds, and replaced after a fresh coat of paint (stressed to appear weathered). Use older materials (or old looking), use minimal effort, and keep polished professionals out of the mix (unless they are capable of slightly shoddy or hurried work like the kind that comes with working with a beer in hand). Keep the markings of family history, especially those pencil lines measuring the growth of generations on the kitchen door jamb, and the cutesy painted signs, but repair the inevitable rot that seeps in, seeking to destroy all history.
There is a term for all of this: cottage life. It means paring down to the essentials you can fit in one bag (never a suitcase, which has implications of a hotel) and always having a good book or crossword puzzle to kill time with or signal that you are having alone-time.
Pitching in for meal prep or clean-up (choose one) without question is required. Dishwashing machines are forbidden in cottages since washing dishes is another way for family members to share in the collective industry, one washing and one drying. The kitchen is always small, requiring dance moves to navigate around one another, but it is wholly adequate, and the utensils are spare but serviceable. Most cottages have a few months set aside for seasonal rentals, so nothing of great value is kept there although everything there has sentimental value. Family history is written in the worn edges and missing parts: the knife that grandpa used to clean fish, the cutting board that is bowl-shaped from a half century of use, or even the trinket on the windowsill—God only knows where it came from—which no one dares to get rid of at the risk of disturbing the delicate balance of cottage hierarchy.
There is no visual way to capture cottage life. Photos won’t do. The family cottage is more of a scene than a structure, more of a shared history than individual experience. You can’t buy into it, and it can’t be sold. Sadly, many cottages do have to be sold, due to rising taxes; but hopefully, the new owners realize the value of their acquisition and tend to the history of the place with care.
Even with all a cottage has to offer, like the family cottage Scarlet has been visiting since childhood, the addition of a sauna is like ice cream on the apple pie. Not completely necessary, but it sure is a good combination. Saunas are gathering spots for communal experience. They enhance the routine of cottage life—morning sauna, evening sauna, all-day sauna—and their heat extends the swimming season indefinitely. On Cayuga Lake, the swimming season is typically two or three months (depending on how much polar bear is in your DNA). With a sauna, lake dipping is possible all year!
So, with all of this in mind, we recently converted a small room in the walk-out basement of the cottage into an electric-stove sauna. Never mind that the floors above all sloped several inches in ten feet or that the headroom is barely six feet, we worked with what we had and created a perfect oasis of heat. Some family members will have to work on their bad posture to avoid bonking their head, but once on the bench—whose height is always measured from the ceiling down—all is well. An existing steel framed basement hopper window was converted to cedar and now frames a perfect view of the lake. The 9 kW Harvia Cilindro heater with its two hundred pounds of rocks holds the heat to make the sauna usable for hours with repeated löyly.
Now that it is mostly done (cottage projects never seem to get finished), we have been using it daily while there. It is making our late summer stays at the lake perfect. I can’t wait to come out later in the season, or even in winter, to enjoy a jump into the lake and tease my inner polar bear.
I love physics because it deals with the tangible effects of the forces of nature, the interactions between matter and energy that explain the things we feel or see daily. In this post, specifically, I want to delve into the transfer of heat, which seems to be a hot topic in sauna forums.
There are three methods of heat transfer: conduction, convection, and radiation. In a sauna and everywhere else, unless you live on a planet at absolute zero (-460 °F), all three types of transfer exist. Heat always goes from a warmer object to a cooler one, and closed systems are entropic, that is to say, if you sip too slowly, the ice will melt, and your cocktail will eventually be at a lukewarm room temperature. The transfer of heat is greater when the temperature difference, Delta T (ΔT), is greater. In addition, it slows over time, until the system’s temperature equalizes, which, for our study, includes not just inside the sauna, but the environment it sits in. Meaning, no matter how well you insulate it, the sauna will eventually reach the ambient outdoor temp, unless you keep the heat on as in a house (or a sauna in a house). This is a factor in freestanding sauna design. We must assume the starting point is anywhere from 0 to 100°F (unless it is fired up constantly) and the desired bathing temp is 180-220°F. In a house, we are trying to hold the temp at about 70°. In the residential sauna, we need it to hold temp for a few hours, at the most.
Conduction is the transfer of heat from one solid or liquid to another by direct contact. Grab a (foolishly installed) metal sauna doorknob that is either 200°F leading into the hot room from the dressing room, or 10°F coming from outside into the dressing room (depending on the season), and the heat will rapidly conduct either to your hand or from it, followed by your shriek. Same is true if someone bumps you against the hot stove as you leave the sauna, burning your butt to the point where sitting is impossible for two weeks…as happened to me once. This form of conduction is typically avoided in the sauna, but it happens. Less dense materials, like your towel, mitigate conduction. This is why we look for low density boards like cedar, not hardwood, for the benches. Black walnut would feel like a hot iron on your posterior.
Convection is the transfer of heat through the movement of fluids. It is in part driven by gravitational forces; whereby, warmer gases or liquids, which are typically less dense and lighter, tend to rise while cooler ones sink. This creates a convective loop as the heat is circulated to the walls of the room, for instance, or to you on the top bench, at which point the air cools and falls, creating an endless loop. I say typically, because there is this oddball exception: water close to freezing gets less dense and thus freezes on the top of a lake or pond, making hockey, ice plunging after a sauna round, and life on this planet possible. If the movement of air is stopped, say by the fibers of mineral wool or two close layers of glass, it becomes an insulator. Air itself holds very little heat per volume (more than a thousand times less than water); whereas, water holds twice the heat energy of granite and about the same as steel. A large volume of this dense heat-holding material is called a thermal mass. By acting as a reservoir of heat, this mass can mitigate the fickle effects of convection, especially when the air is coming and going. This is why we try to keep the door closed in the sauna: The air convection that swirls invigorating heat around us is disturbed by the cold air rushing in to replace it. But that’s not so bad. We want some fresh air circulating and enough thermal mass to mitigate the swings in temperature.
In home construction, the emphasis is on controlling convection: eliminating it inside wall cavities and not allowing warm air to escape from heated (conditioned) spaces. This is especially necessary up high where warm air creates a chimney effect; whereby, escaping warm air creates negative pressure and draws in cold air from wherever it can. In a not-so-old house on a cold night, put your hand over the wall outlets, even on interior walls, and you will likely feel cold air being sucked in. More so if you have a big, cozy, romantic fireplace with an actual chimney and a roaring fire, which feels great but pulls the heat right out of your house.
In a freestanding wood-fired sauna, there will be leaks and cold air coming in. Again, that’s ok because we want fresh air, as long as we control where it comes and goes. Air and steam will move the heat around, but eventually, it settles into strata: hot up high and cold down low. Air movement can help break up this layering of cold to hot, but it is difficult to control. Thus, the upper bench will always be hotter, unless you have an Aufgussmeister to move the heat around with his swirling towel dance.
The last method of heat transfer is radiation. Sounds bad, like Chernobyl, but radiation is everywhere. All objects with a temperature above absolute zero (−459.67 °F ) emit thermal radiation, mostly in the infrared range that we can see with a special camera. At a certain point, heat becomes visible light, and the color of the light corresponds to a specific temperature. The dark red glow of a poker in the fire (or the top of my sauna stove when I fire it hot) is 1200°F (these are specific colors-blacksmiths, for example, will have a color chart on their shop wall from dull red to bright yellow). The surface of the sun burns at 5772° Kelvin, which is the color of the sunlight we bask in on the beach. Fortunately, the sun is far away and appears relatively small; otherwise, we would burn up instantly. The human body radiates heat as well. After getting sunburned, your skin will be hotter than the person next to you and will radiate heat to them. In fact, all bodies, especially black bodies1 (which are not necessarily black), radiate and absorb heat, depending on which is hotter. The only things that are not black bodies are things like foil, which reflects most heat directed at it. Surface area and angle of incidence also matter: The more surface area and the more parallel two surfaces are, the more heat transfer. Temperature difference matters as well. Too much difference and the effect is intense, like when I pour bronze and have to stand an arm’s length away from the pot of molten metal, or when I stand on a subzero surface in winter and feel the heat being sucked from my body. Too little difference in temperature (ΔT), and radiation is hardly noticeable. Direction is also important. The fireplace heats our front but not our back. I have a story about a cold, drizzly camping trip when all my companions and I could do to stay dry was to keep putting our jackets on backward then forward as we sat by the fire. And in all these situations, it is aluminum foil that saves the day: as an apron to wear, a foil surface to stand on, or an emergency blanket over the shoulders. Foil blocks radiation, but it needs an air gap, lest it become extremely conductive. Without any barrier, heat—like light, radio waves, and the rest of the electromagnetic spectrum—can radiate millions of miles. Those episodes of Leave it to Beaver from your parent’s childhood are still traveling through space.
In the sauna, radiation is crucial as it creates an enveloping heat that comes at the bather from everything hotter than 98°F (body temperature). If the whole room—walls, benches and rocks—is 200°F or more, we will feel the heat coming from each of those surfaces. Colder surfaces like a big window or that guy that just got out of the cold plunge will suck heat from us. Something too hot, like a blasting fire in a single wall stove pipe, will feel searing. In an electric sauna, the rocks need to cover the heating elements, so we don’t see/feel the searing red heat. The much cooler, but still hot rocks will then re-radiate the softer heat. Foil behind a cedar wall (or other wood) will reflect interior heat leaving the building back toward the cedar which will re-radiate toward the interior. The walls need to be just so hot. Radiation also mitigates the effect of the constantly changing air. The air may be cool, but the radiation of the hot surfaces will cut through the cold like the winter sun on your face. (Speaking of which, there’s nothing like a full-body sun bath on a calm, freezing day to boost the sauna experience!) The thermal mass mentioned above will continue to radiate heat even if the door is left open. Cool air swirling in will kill the radiation buzz for sure, but as soon as the door is closed, that warm fuzzy feeling will come back.
So how does all this daydreaming back to high school physics class inform how I build my saunas? A lot. I want the radiant heat off the stove to work for the bathers, warming them just so, like the sweet spot in the campfire where campers should roast a skewered marshmallow (but never do). I aim for a soft radiant heat with a ΔT of a few hundred degrees at most (the bather: 98°F, the rocks: 400°F); an omni-directional heat, which gets all the walls and benches up to 200°F before sauna time; and a not-too-intense heat. (Make sure the fire has died down, and the stove pipe, if single wall, is not too hot.) A big window is pleasant to look through, but it must not be too large, as it will suck the heat away from bathers, and a cold cascade of negative convection will sweep over the floor. Thermal mass is great, but again it must not be too substantial because the sauna will take forever to heat up, and no one seems to have the time for a daylong sauna ritual as in the days of old.
I have my bathers facing the rocks. Typically, the stove is fired from outside, so there is no worry about the intense (visible) radiant heat through the firebox glass door. As cozy as that sounds, it may feel too much like sitting around a hot campfire, and that is not the quality of heat you want in a sauna.
Recently, in an online sauna forum, I read two seasoned sauna veterans stating, “you don’t want radiant heat in a sauna.” I believe they misspoke. High intensity radiant heat does not belong in a sauna, but a lack of radiant heat is only possible if all surfaces, bodies of mass, and liquids have reached a state of equilibrium. That is to say, equilibrium can be reached in a sauna of 100°F or when it is as hot as the rocks, in which case, the bather is cooked like a goose. As long as the bather is cooler than the rocks, stove, walls, and benches, heat will radiate to them. It is said that when you close your eyes in a good sauna, you cannot tell where the stove is.
How do we get there? Install radiant foil behind the wood walls (with an air gap) so the foil can reflect heat back into the wood and back into the sauna; use a high-rock-capacity stove or heater (thermal mass) to hold and radiate the heat; fire the kiuas (stove) hot to get the rocks and the whole sauna deeply heated, but let the intense fire die down before bathing; and make sure everyone faces the stove, so the radiant heat (which travels as waves, like light) reaches everywhere.
You can always tell when a sauna has good löyly; everyone coming out looks so… radiant!
The latest sauna we built is an indoor electric affair. We installed it in a new addition that also holds a hot tub and pool, an enviable combination for a personal home spa. It has an ample window and large 10 kW Harvia Cilindro heater that should make the top bench a real hot spot. I love the view from that bench. Not just any view, but one that takes me back to my childhood.
The property is located on the eastern shoulder of Cayuga Lake, at a point where the land starts to dip dramatically downward to the long snaking shore. The slope is so steep here that you don’t see the lake, only the opposite side a little more than two miles away. Someone unfamiliar with our landscape might not be aware that the longest of the Finger Lakes fills a glacier-carved trough below. While the scene through the window feels close, it is in fact, a long journey away.
The most pronounced land formation is Rice Hill: the old skiing and sledding hill in Trumansburg [or Ulysses?] that at one time had a rope tow run off an old tractor motor. There’s a warming hut and two shallow ice rinks at the top of the hill where many a hockey game was played. When friends and I were too broke or didn’t have the ambition or means to go to the closest downhill ski area, we would go to Rice Hill and practice our S turns. I also recall many tobogganing adventures; it was the kind of hill where serious injuries marked a good run.
Just to the north is a ten-acre parcel my parents bought in the 1960s with the dream of building a house. My dad designed the home with all the meticulous detail he employed on his large-scale architectural projects. It was a three-story modernist affair with a flat roof and cantilevered balconies that would have commanded a view across the lake precisely to the location of this sauna.
In the early ‘70s, things turned south for my dad: there was a recession, he lost his job, increased his drinking, and the dream of the modernist masterpiece overlooking the lake deflated like a balloon the cat clawed. All we were left with was a model of the house my dad crafted out of mat board, with twigs as stand-ins for trees. Later, after I finished grad school and before my parents had to sell the property as part of a bankruptcy plan, I lived on the lot in a tent and tarp shelter, waiting for my dreams to come to life.
I bring all of this up because of the prevailing association of sauna with memory. So many of my clients, who are typically aging baby boomers like me, say they want a sauna because of the wonderful childhood memories they have of sauna time.
Perhaps their family has Finnish roots, and they experienced summers in Finland, or they had a camp somewhere with a sauna. Like my experiences at Podunk, these childhood memories start to loom larger with age. Memory acts as a filter; the important things are retained and the trivial is set aside.
Landscape acts as a placeholder for memory. Living later in life where I grew up, I constantly encounter places that stir memory. While working on this project, with my past literally visible beyond the window and across the lake, I was constantly reminded of my connection to this magical place in the heart of the Finger Lakes. Sauna is a keystone in all of this.
As with so many saunas I build, I stop work and dream for a moment; what if it was mine? But then, I hand over the sauna to the new owners so they can ponder their own dreams. In this case, the owner will gaze out the window at the immediate surroundings: Land that he grew up on.
Traveling with one of our mobile saunas to the lake and delivering to new owners.
We have been building mobile saunas for the past ten years! And because of their growing popularity and versatility, they now make up most of our business. Each unit is still handcrafted with many layers of details. The owners each get a unique product tailored to their specifications. It’s also an easy way to avoid zoning and permitting restrictions and the hassles of a site-built project. Investing in your dream sauna makes more sense if you know you can take it with you!
We hear from our customers all the time that their friends, family, and neighbors are as excited as they are to have a sauna!
Mobile wood-fired sauna, on a trailer.
For many people, owning a sauna feels like a bare necessity during the winter months!
This recent sauna build is just steps from the owner’s historic, New England homestead. Our saunas are designed to blend in with the home and environment. The classic details of this historic house are elegant rustic with dramatic rock outcroppings and a fire pit, making for a perfect gathering spot.Our wood-fired mobile saunas travel well and can be parked in beautiful places.Scarlet by the lake ready for a cold plunge
Having a sauna at home is a low maintenance life upgrade.
Our 5’x8′ mobile sauna parked in town with 100 gal. cold plunge tank.Interior of our larger mobile sauna. With a large pile of rocks, our saunas can get as hot as you like. We aim for 212° F (100° C)We build many mobile saunas in our shop in Ithaca, NY. Working in our 3000 sq. ft. shop is more efficient than building on-site.Two sizes of mobile Saunas on display at our shop in Ithaca, NY
We offer building plans for DIY sauna builders or your local builder for one-time usage only. Thanks to our valued sauna plan customers, and the growing popularity of DIY sauna building, we have taken the opportunity to launch our new & improved mobile sauna building plans! Our sauna plans are fifty plus pages and include detailed notes, drawings, photos, and material lists for a wood-fired, 5’x8’/6.5’x10′ build. If you are thinking about purchasing our plans or building a sauna, we offer you an opportunity to build your own sauna using construction plans. Rob Licht has developed the best practices of sauna building with thirty plus years of experience.
If the heart of the sauna is the kiuas, or sauna heater, then the soul is the rocks.
Every brand of sauna heater I’ve installed has a different approach to the rocks. Some, like a wall hanging six or eight kilowatt unit use less, maybe forty pounds. And some, like the Harvia Cilindro or Club heaters use more, up to two hundred pounds. Wood burners vary too. Kuuma’s heater takes one hundred fifty pounds or so, but you have to provide your own. My custom-built stoves (Lämpimämpi brand) take a similar amount of stones and like the Kuuma stove, the rocks are piled on top, mounded as high as you can. The Harvia Legend, an elegant wood burner from Finland, has a basic cage surrounding the stove. By the time you are finished loading the stones, you won’t see the stove. There is also an optional cage that surrounds the stove pipe that hold even more rocks.
The Finnish and Swedish heaters use olivine diabase or peridotite, igneous rocks found in Scandinavia. The rocks I have used that come with the electric heaters look the same: grayish chunks, some flatter, some chunky, all in the size range you’d find in a bag of potatoes. These are amazingly cheap, considering that they come all the way from some quarry in Finland or Sweden. The UPS driver stopped asking what was in the boxes (“yup, rocks”). Once, I found a note in one from a young Swede, hoping that I was happy with my rocks.
My favorite rocks are hand-selected glacial erratics: various igneous granitic and metamorphic stones that I find in places where glacial melt waters sluiced out potato-sized rocks.
These are heavier and denser than our native shale, which is worthless for the sauna since it tends to explode when heated. The erratics have a crystalline structure that you can usually see and a solidity and heft evident when you pick them and bang them together (they are not easy to chip).
My favorite rocks.Lämpimämpi with glacial erratic rocks.Lämpimämpi with jagged olivine diabase rocks.
A significant consideration when selecting a heater is the rock capacity. The more rocks, the longer you should let your sauna heat up. Clients always ask: “How long should we let it heat up?” While I usually say, “About forty-five minutes,” I really want to answer with the same sort of retort I give when someone asks how long it will take for paint to dry (“as long as it takes”). The correct answer should always be: the sauna is ready when the rocks are hot enough to produce good löyly or steam. Löyly is the soul of the sauna’s mysterious expression. And how hot is that? Four hundred fifty degrees Fahrenheit, more or less. If you want more steam, then you want more rocks.
Löyly is the soul of the sauna’s mysterious expression.
More rocks are good If you expect several rounds or people coming and going. No one wants to pour water on the rocks expecting a stimulating burst of steam (but not too sharp), only to hear the fizzle of water barely boiling. That’s like trying to make an omelet in a pan that is barely hot. Disappointing, to say the least, but also bad for the heater. There’s a Finnish saying that goes something like this: when you leave the sauna, put another log on for the sauna elf, or he will pee on your stove. Which means don’t keep throwing water on the stove, especially at the end, if the rocks are not hot. The warm water will just sit on the steel stove and rust it. When you throw water on the rocks, it should all turn to steam, and when you are done, the stove should dry quickly. When I finish eating my omelet, I rinse my cast iron skillet and let it dry over the flame; I don’t let it languish all wet in the dish rack. The same principle applies to the sauna stove.
Jagged rocks will catch and momentarily hold more water, allowing less to get past the rocks and into the heater. The dense, roundish rocks I use produce a nice soft steam, but they have to be hot. I’m not so worried about water getting down to my stove, which is always very hot. (Lämpimämpi stoves have 3/8” steel plate on top so they will never rust out.) By the way, always use clean water and be wary of city water with chlorine or even country water with minerals or methane. We use only use filtered water or rain water for löyly at home. Spring water or distilled water might be prudent in some cases. Never use pool water or, worse, let some disrespecting novice wring out their bathing suit over the rocks, as I have seen in a gym—pleasant smells should emanate from the rocks. Special sauna oils or even Sauna Brew can be added to the löyly water for aromatic effect. Heck, we used to add beer to the water (cheap lager was always best).
Custom sauna with Harvia Cilindro and carefully placed olivine rocks.
If your electric heater comes with the jagged, grey olivine rocks, I suggest using them. They are selected to fit between and around the elements. When setting the stones, first I rinse them, then I take care to arrange the rocks one by one. It can take me forty-five minutes to fill a large capacity heater. For the Cilindro, I place large rocks at the bottom to hold up the others and then work a flattish layer against the cage wall. I then backfill against the elements with smaller and irregular rocks. On the smaller, wall-mounted units, I try to fit the flat stones between the elements without forcing or bending them. The goal is to loosely fill the cage so there is good airflow and to completely conceal the elements. Lastly, I top off the heater with larger stones and try to arrange them so they will catch water and force it inward, not splashing it out. In Finland there might a “spirit stone” placed here: a special stone or stones from a favorite place (make sure it is igneous or metamorphic—test it in a campfire first if not sure). The top might even have a layer of more decorative stones, which in Finland you can buy from sauna dealers, but here in the US, I have only seen the same dull grey stones. The Harvia web page has a good section on stones and how to place them into the pillar.
Once you have the sauna going and have been using it for a year or so, expect the stones to settle and flake off. You should remove them, vacuum out the heater and replace the rocks loosely, discarding the broken ones and adding more if needed. Failure to do so will cause the heater to eventually overheat and lead to the elements failing.
Care in selecting and placing the rocks will ensure that your sauna…Rocks!
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