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!
Having grown up around and in the waters of Cayuga Lake, whose long finger touched the shores of my childhood stomping grounds, it is no wonder that my dreams often turn to things nautical. When my friends and I first started taking saunas at Podunk in our early teens, the fantasy topic of floating saunas came up frequently. We loved swimming in the lake, but its waters are only warm enough for swimming from the beginning of July to about mid-September. What better way to extend that season than with a sauna? A sauna on a boat! Sweating not just near the water, but on the water. (Oh, but we did enjoy running naked to the creek!)
I’ve had an ongoing affair with boats. Especially a love of canoes that goes back to my discovery of the Adirondack waterways that form an almost continuous route from civilization into the deep wilderness and back (the caveat being that short carries are required).
I started making a boat years ago: a strip canoe affair. Not strip, as in naked, but strip as in thin bands of cedar, all joined and sandwiched between two epoxy and fiberglass layers. I never finished that boat; its progress was aborted midstream after I broke my collar bone in three during a trail running race. The unfinished shell still looms over my shop as a reminder, high up in a loft space. So, suffice it to say, that when a client approached me about building a much bigger boat, I had my hesitations.
Mark initially wanted a beachfront sauna. Then zoning and other issues steered us to thinking of a floating sauna. My childhood fantasy! Granted it is not a new idea; in fact, there are several in Norway and other places. But on Cayuga Lake? This was to be a first. It made sense, in a fantastical way. He had ample dockage but limited beach. He was willing to invest in the idea and take the risk, and he was a nice guy, with just enough chutzpah to make it happen.
Client Mark and Family on Maiden Voyage of Sauna Boat, Cayuga Lake, New York.
The design phase took over a year. It was a real challenge because it entailed not just carpentry but nautical engineering. Precision was required, and my hand-drawn methodology needed some sharper pencils. Some thirty pages of drawings later and we were ready to build. We had a great fabricator for the frame, ladder-stair, and railing (Service Machine Tool in Elmira, NY) and some other great help along the way, but the whole thing—all twenty-six feet of it—was assembled in the shop. It was a predicament as the beast took over, floor to ceiling, and there were threads of self-doubt along the way to trip over.
I am not a boat builder, so there was as much learning as doing, but we pulled it off—including the challenging assembly of four round cedar windows trimmed with authentic ship-salvage portholes.
There were a lot of other finicky details. (I’ve come to understand that boat building is all finicky details.) The biggest challenge was loading and transporting the beast on an oversize low-boy flatbed truck. At one point, we had the 10,000-pound hulk levitating on three forklifts as the low-boy flatbed backed under it. The guys at Lansing Harbor Marina gave us confidence, especially after it passed its initial float test. After a few months of tweaking, we took the maiden voyage, complete with a champagne toast.
A unique feature of Mark’s sauna is that it is a fully navigational boat with twin electric motors. The sauna is heated with a gas-fired heater and has 12-volt electric lights powered by a solar-electric system. Ideally, it will be used on calm days when friends and family can drift out to the middle of the lake, experience sauna, jump into the clear waters, cool off on the roof deck, and repeat until the fantasy has been fulfilled. Maybe even under the stars or northern lights.
Thanks to everyone who helped make this possible. Thanks especially to Scarlet, who believed in the dream, and Mark and Karie, who supported it.
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