1. Smaller is better
A sauna sized for its everyday 2 bathers warms faster, holds its heat and needs a smaller heater. Guests fit on the lower bench a few times a year.
2. Multiple bench levels
Heat rises, so a bench 45 cm higher is a hotter sauna. Several levels let a beginner and a regular sit in the same session, each at the heat they want.
3. Head near the ceiling
Heat pools under the ceiling, so the upper bench goes 1.2 m below it and your head sits in the heat. A tall room is heat you paid for and never sit in.
4. Wood where you touch
Soft, light and knot-free where skin lands. Low density stays cool at 90 °C where a dense board burns, and a knot or a run of resin bleeds when hot.
5. No metal where skin goes
Screws from behind, or countersunk and plugged; bench slats on hidden fixings. A screw head at 90 °C burns the skin, and so does a hinge or a rail.
6. Door opens outward
It is the only way out of a room at 90 °C, so it opens with a push and never locks: a magnetic or roller catch, not a latch, and tempered glass if glazed.
7. Stone floor, with a fall and a drain
Löyly water and sweat end up on the floor, so it drains to one point. A duckboard on top keeps the feet off the stone; wood on the floor itself rots.
8. Insulation, foil, and air gap
50 to 100 mm of insulation, foil with taped joints as vapour barrier, then boards on 10 mm spacers so they dry from behind, the lowest 100 mm off the floor.