How a proper Finnish sauna works, so you can build your own.
Do not build a big sauna if 90 % of the time only 2 people will use it. It costs more to build, takes longer to warm up, and needs a bigger heater to do it.
Heat stacks under the ceiling; every centimetre above your head is volume to heat for nothing. Heater manuals specify 2.1 to 2.3 m, with 1.95 as the minimum.
So your head sits in the hot layer. The lower bench a step down, at 45 cm, for the feet and for getting up.
The whole point of the heights and the ventilation. A sauna with cold feet is a badly built sauna, however hot the ceiling.
50 to 100 mm of insulation, aluminium foil as the vapour barrier with the joints taped, a 10 mm gap on spacers, then 12 to 16 mm panelling. The gap is what lets the wood dry.
Cedar, aspen, alder, hemlock: low density stays cool to the skin. No knots on the benches, no resin at head height.
Screws from behind, or countersunk and plugged; bench slats on hidden fixings. A screw head at 90 °C burns.
Add 1 m³ to the room for every square metre of uninsulated glass, tile, brick or stone, the heater will be heating the wall. Undersized heaters run flat out and never make löyly.
It is the only way out of a room at 90 °C. A magnetic or roller catch, never a latch; glass tempered.
Löyly water and sweat end up on the floor. A duckboard on top for the feet; wood on the floor itself rots.
Air must reach the back of the panelling and the boards must clear the wet floor, or the wood stays damp and darkens.
Coming soon.
One formula for all seventeen layouts, so the gaps between the numbers mean something: 100 against 42 is the fan itself; 42 against 19 is where the inlet sits.
Score = breathing × feet, 0 to 100. It is a model, like everything on this page: it orders the layouts and sizes the gaps between them; it does not measure a sauna.