Thermowood is timber that has been heat-treated at around 400°F in a low-oxygen kiln, without chemicals. The process removes moisture and breaks down the sugars fungi feed on, leaving wood that is more dimensionally stable and more resistant to rot than untreated softwood. It is darker in colour and slightly more brittle than the original timber.
Why it matters when you are comparing products
For an outdoor sauna that lives in the weather, dimensional stability matters more than appearance. Thermowood is a construction choice worth paying for on a shell exposed year-round.
Where this comes up
- Outdoor saunas — Where wood treatment does most of its work.
- Indoor vs. outdoor sauna — What changes when the shell lives outside.
Reviewed 2026-08-15