Thermowood in Saunas and Steam Rooms: Why This Wood Holds Up

Saunas and steam rooms are among the toughest environments wood can face: air temperature in a dry sauna can reach as high as 90 degrees Celsius, while humidity in a steam room is almost always at saturation. Ordinary wood warps and cracks over time under this cycle of heat, cold, wet, and dry. Thermowood was designed specifically for exactly these conditions.

What happens to thermowood under heat and moisture

In the thermal process, wood is heated to a high temperature, roughly 180 to 212 degrees Celsius, in an oxygen-free environment. That heat breaks down the sugary compounds inside the wood that normally absorb moisture; as a result, the finished wood absorbs less water, swells less, and stays more stable under high heat. That's why, unlike raw natural wood, thermowood doesn't warp or release odor or resin under sauna-level heat.

• Lower moisture absorption means less swelling and shrinking under constant steam
• Higher heat resistance than raw wood, with no need for extra coating or resin
• No release of heated resin compounds — important in the closed space of a sauna

Which wood species works best for a sauna

Lightweight species such as thermally treated pine or spruce are the usual choice for sauna wall cladding and benches, since they transfer heat more slowly and won't burn your skin on a hot bench. For flooring and surfaces closest to the heat source, denser species such as thermally treated alder are also common. The key point is that wood density and species need to match the distance from the heat source — you can read more about these differences in our comparison of thermowood and natural wood.

What thermowood doesn't guarantee in a sauna

Thermowood is more moisture-resistant, but it is not completely waterproof. In a steam room, where surfaces stay wet all the time, you still need proper ventilation and a floor slope to drain water away — otherwise even thermowood won't tolerate standing moisture over the long run. Metal fittings such as screws and clips also need to be rust-resistant: the wood has been upgraded, but if the connecting metal rusts, the old problem comes right back.

The right wood is half the path to a durable sauna; the other half is proper ventilation and floor slope.

Caring for thermowood in a sauna

Unlike ordinary wood in damp spaces, which needs fresh oil or wax every year or two, thermowood in a sauna usually needs much less upkeep; you just need to ventilate the space after each use so any remaining moisture can dry out. For exterior walls or similar spaces where moisture combines with sun and rain, the wood behaves a bit differently, as we explain in our thermowood terrace flooring guide.

The bottom line

For saunas and steam rooms, thermowood is a more reliable choice than raw wood, provided the species is matched to its distance from the heat and the ventilation is properly designed. For advice on the right material and execution for your own project, start with our services section.