A sauna is the one interior where timber cycles continuously between 80–100 °C and high humidity. In those conditions material choice stops being a question of comfort.
Why resin is a problem
Resinous species such as pine and spruce push resin to the surface as they heat. Hot resin is sticky and carries a burn risk on contact with skin. That is why resinous timber is never used on seating or backrest surfaces.
Thermal conductivity
A dense, hard timber conducts heat quickly — and burns whoever sits on it. Sauna benching needs a low-density species that conducts slowly. Ayous is the standard for exactly this reason: resin-free, light, and it does not feel hot to the touch.
Surviving the moisture cycle
Sauna timber wets and dries several times a day. A dimensionally unstable species checks and opens at the joints within a season. Thermowood Ayous or Thermowood Pine move markedly less through that cycle.
The practical rule
Ayous for benches and backrests; Ayous or Thermowood for wall and ceiling panelling; a hardwood rated for wet floors underfoot. And never varnish or oil the surface — a sealed face traps moisture inside the timber.
