Saunas 101

How a proper Finnish sauna works, so you can build your own.

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How the score is built. Breathing × löyly × heat, scaled from a sealed room (0) to the best layout (100). An open drying vent is for after the session.

Breathing is the CO₂ at the upper bather's mouth: full at about 970 ppm, the best class of EN 16798-1, and zero at a sealed room's.

Löyly is the share of the heater's rising, steam-carrying air that comes down to the benches instead of escaping at the ceiling.

Heat is how much of the room's heat the bathers sit in, from ankles to mouth. A ventilated room that leaves the benches cold is not a sauna.

The air is a pressure balance, not CFD: 125 mm openings, a door with 2 mm gaps and a 120 mm fan, scored running. A duct that starts cold can run backwards and is scored that way.

Score100/ 100Best choice
HEATER UPPER BENCH LOWER BENCH
fresh air heated air stagnant / CO₂
Ventilation
Inlet
Exhaust
Fan
Duct
Heater
Inlet angle
Drying vent