BoulderSauna  Tour the Finnish-ed Product

What's a Finnish Sauna?

A free-standing oasis.

A Finnish sauna is an insulated, vapor-proof box internally lined and accessorized with a heat-tolerant wood such as western red cedar or redwood. It includes benches to sit and lie on, a vapor-proof light, an adjustable air vent, and of course, a powerful heater (electric, gas, or wood-fired) that can bring the ambient temperature up to around 200 degrees F. You can add humidity from time to time by pouring water onto the rocks that sit atop the heater. Some heaters include a solid pan beneath the rocks that you can fill with water for continuous steam. In Finland, nearly everyone owns or has access to a sauna; to a Finn, not having a sauna is like not having a refrigerator.


A sauna can be a free-standing outdoor structure or more commonly, it's a converted room inside your house. Regardless of location, all saunas require an insulated vapor-proof frame; the free-standing version just adds the need for a foundation and weatherproof shell. Other location considerations include proximity to the outdoors (for rolling in the snow or jumping in the pool during breaks) and to a shower (for rinsing off all the toxins you just sweated out).


Internally, a typical sauna might be six feet wide by five feet deep and 72 inches tall. This size allows one to lay flat on a bench or four adults to sit in reasonable comfort. The low ceiling is important for efficiency — no sense sending heat up to where it won't be used. Likewise, there's no sense in having a sauna larger than you need because the extra space, while luxurious, adds to your construction cost, operating cost, and preheat time.


Fresh air enters under the door and exits to the outdoors via this sliding-door vent set high in the back wall.

Although it may seem contrary to a sauna's primary goal of trapping heat and vapor, there must be some form of controlled passive ventilation to protect you from suffocating. An intake vent can be a gap under the door or a vent in the wall beneath the heater. An exhaust vent can be located in a wall opposite the intake and higher up — either just below the upper bench, above the upper bench, or in the ceiling. The thermodynamics inside the sauna drives the ventilation, so you don't need a fan. A simple wooden sliding door at the exhaust vent allows you to control the ventilation rate.

If your sauna is inside your home (not free-standing), do NOT vent it directly to the outside, rather, be sure the exhaust vent sends the air back inside your home. The reason for this may not be obvious: if you vent indoor air to the outdoors, that lost air can only be replaced by air from the outdoors. If all the windows in your house are closed, your sauna will stop venting because it can't suck in the replacement air — this is dangerous. If you do vent to the outside, then you must either also have your intake vent connected to the outdoors or always open a window near the intake vent whenever you operate the sauna.


This sauna has select tight-knot (STK) cedar in the walls, clear cedar 2x4s for the benches, and clear 1x4s for the duckboard floor. Note how the clearest of these STK panels were reserved for the area just above the upper bench.

Just about everything inside a sauna (the heater, light, and floor are exceptions) should be made of wood — any plastic might melt and outgas, and exposed metal creates a burn hazard (obviously rustproof metal nails and screws hold everything together, but they should be hidden or covered). The most practical wood to use is western red cedar, which, of all woods, holds up the best against temperature cycling, humidity, and even sweat stains. Redwood is another (more expensive) option, but it stains more easily. Some saunas are made with white cedar or other woods, but these are more prone to cracking, warping, and rot, not to mention becoming overly hot to the touch due their higher densities.

The most common wall lining is select tight knot (STK) 1x4-inch tongue-and-groove western red cedar panels. You can pay more for clear cedar lining, but I think the knots are pretty. The benches, on the other hand, are made from clear cedar (usually 2x4's or 2x2's) because knots can get quite hot and you wouldn't want to sit on one. The STK wall panels do include some boards that are clear or nearly so; you can position these near the benches and use the knotiest boards on the ceiling and below the benches. Doors may be solid cedar with or without an insulated window. Sauna doors can also be entirely glass, with cedar handles. A cedar safety fence always completely surrounds the heater. The small area of floor that is not directly under a bench or the heater may covered with cedar decking, but one can also leave it as bare concrete or any other water-tolerant material (the floor will never get hot, even when it's 200° F at the ceiling. All of the cedar is left untreated and unfinished as is customary and important for air quality — any finish or treatments are unnecessary and would outgas and quickly deteriorate.

Heater & Light

The cleanest and most common way to heat your sauna is with an electric heater, usually powered by a 240-volt dedicated line. Wood- and gas-fired units also exist, but these require more complex installations and they can affect air quality inside the sauna. Manufacturers generally offer a range of electric models varying in wattage, quality, durability, and features. All include a pile of rocks suspended on a grill over the heating element. Standard heaters let you toss water onto the rocks for steam, and any water that doesn't vaporize just drips down around the heating elements and onto the floor. Deluxe models include a humidifier: part of the rock tray is solid, allowing you to fill it up with water for continuous steam.

This is a 6000-watt, 240-volt, all stainless steel heater by Sauna Craft. Hidden beneath the center of the rock pile is a water reservoir for continuous steam.

This reservoir holds about a liter.

The heater control panel includes a temperature setting and a timer. Controls may be analog (physical dials) or digital (push buttons and numeric readouts). Digital controls give you better precision, silent operation, and programming features like delayed start times. Usually the controls are mounted outside the sauna to protect them, but some may be mounted on the lower part of the heater.

You'll need a a vapor-proof ("outdoor") light fixture mounted on a wall or the ceiling. A low-mounted dimmer switch inside the sauna is a nice touch.

Some famous person might have worn these...

Frame and Insulation

A sauna must be well insulated for efficiency and made vapor proof so all that steam doesn't rot your frame — cedar holds up to water quite well, but this is not so for your insulation and wall studs. You can frame it with ordinary 2x4 spruce wall studs filled with fiberglass or eco-friendly recycled cotton batts. With fiberglass, you more or less need a space suit and a decon chamber to install it without getting glass fibers stuck in your skin and lungs; with cotton you can do it naked and not even have to shower afterwards. Between the insulated frame and the interior cedar lining is an aluminum foil vapor barrier covering the walls and ceiling. You have to use aluminum foil because the conventional plastic vapor barriers will certainly outgas into your sauna and probably melt.

If you are refitting a room in your house, you can build a frame in front of any existing walls and ceiling. Alternatively, you can use the existing framing as long as you remove the wallboards and replace the vapor barrier with aluminum.

Exterior (Free-Standing Saunas)

Normal home building practices apply to a free-standing sauna; where on the interior we are concerned about heat and steam, on the exterior we care about wind and rain. The outside of the frame, covering the studs and insulation, is usually a layer of sheathing like oriented-strand board (OSB), followed by a waterproof house wrap like Tyvek or tar paper, and finally, any sort of exterior siding (cedar is traditional, especially if you don't want your sauna to end up looking like a tool shed). Any sort of roof will do — asphalt shingles, polycarbonate, aluminum, even tile — just make sure the attic is vented to help it handle all the heating and cooling cycles and to allow any unexpected water vapor to escape.


This floor is made from concrete pavers set into gravel. The wall panels end about one-half inch above the floor — you can see the bottom edge of the foil vapor barrier peaking out of the gap.

The floor can be any water-tolerant material such as you would find in a kitchen or bathroom. The sauna floor won't get hot, so it can be rubber, tile, concrete, even dirt. An outdoor structure typically rests on a concrete pad, a stone patio, or concrete pavers. Inside, you have to be careful to make sure the floor is sealed to prevent water from penetrating to any supporting frame. You may want to line the floor with cedar decking, often called "duckboard."

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