A sauna uses timber for two entirely different purposes, and the timbers that suit those purposes are not the same.
Structure versus contact
The shell, framing and cladding are structure. They carry load, resist weather and hold the vapour barrier. Nordic spruce is excellent here: it is strong, straight-grained, widely available in long lengths and stable once kiln-dried to 12 %.
Benches, backrests, door handles and headrests are contact surfaces. They touch bare skin at 90 °C. Here the governing property is thermal conductivity, and spruce is the wrong answer — it holds resin pockets that blister and it conducts enough heat to be genuinely uncomfortable.
Why thermally modified aspen
Aspen has no resin channels and a low density that limits conduction. Thermal modification at 190 to 212 °C in a controlled oxygen-poor atmosphere then does three useful things: it drives out remaining extractives, it reduces equilibrium moisture content to 6–8 %, and it collapses the hemicellulose that would otherwise feed decay.
The result is a timber that stays dimensionally stable through hundreds of heating cycles, stays comfortable to sit on, and does not need any surface coating — which matters, because a coating in a sauna will eventually blister and be tasted in the air.
Where cedar fits
Western red cedar is the third option, usually chosen for aroma rather than performance. It is naturally rot-resistant and pleasant, but it is softer than thermo-aspen, marks more easily and costs considerably more. We specify it for infrared cabins, where operating temperatures are low enough that its softness is not tested.
A short specification rule
Spruce for the box, thermo-aspen for everything a bather touches, cedar when the client wants the scent and accepts the maintenance. Almost every timber problem we are called in to fix comes from ignoring the middle clause.
