What Is Thermally Modified Wood in Sauna Listings?

What does thermally modified wood mean?

Thermally modified wood, sometimes called heat treated wood or thermowood, is standard sauna lumber that has been put through a controlled heating process. The treatment changes the wood's cellular structure, making it less able to absorb water and more resistant to warping, cracking, and decay. In sauna listings, the phrase tells you that the manufacturer treated the wood to withstand the specific demands of a hot, humid room.

Thermal modification is not a wood species. It is a treatment applied to common softwoods like spruce, pine, or alder. When you read 'thermally modified' in a listing, ask whether it applies to the cabin shell, the benches, or both. The term can appear for a single bench slat or for the entire interior. For a deeper look at how different woods compare in a sauna, see our guide to which sauna wood is best.

Why heat treatment matters in a sauna

Saunas get hot. Traditional saunas are typically heated between 150 and 195 degrees Fahrenheit, according to the Cleveland Clinic. The air is usually dry, but many bathers add water to the rocks to create steam, which raises humidity dramatically. Wood that isn't prepared for that cycle of heat, moisture, and drying can warp, split, or release resin.

Benches take the worst of it, because they sit closer to the heater and absorb sweat and moisture from bathers. A thermally modified bench is less likely to cup or crack after repeated sauna sessions. It also resists the growth of mold and mildew, which is a concern in any room that goes from hot and dry to wet and steamy. For more on how moisture behaves in a sauna, read about sauna humidity and temperature terms.

How thermal modification works

The process starts with kiln-dried wood. The lumber is then heated in a specialized chamber with a controlled atmosphere, typically using steam to prevent the wood from catching fire. The heat breaks down hemicellulose, the component that attracts moisture, and it also reduces internal stresses. The result is a darker, more stable board that takes on a rich brown color.

Manufacturers choose the treatment temperature and duration based on the target properties. Higher temperatures produce greater dimensional stability but also make the wood more brittle, so there is a tradeoff. Because the treatment removes the food source for fungi, thermally modified wood is naturally resistant to decay without chemical preservatives. For a dedicated explanation of this process and its uses, see our article on thermowood.

Where thermally modified wood shows up in listings

Thermally modified wood is most commonly seen on interior benches and backrests, because those surfaces are in direct contact with skin and moisture. Many manufacturers also use it for the cabin's interior wall panels, window frames, and door jambs. It is less common on the exterior shell, where weather exposure is the primary concern.

If a listing says 'all interior wood is thermally modified,' that is a good sign, but verify which surfaces are included. Some listings only treat the bench tops and leave the wall panels untreated. The list below shows the typical locations you'll see the term.

• Bench slats and seat tops
• Backrests and leaning rails
• Interior wall and ceiling panels
• Door frames and trim
• Accessories like footrests and headrests

How to evaluate thermally modified wood claims

Not all thermal modification is equal. The treatment level, the wood species, and the thickness of the board all affect performance. A thin bench slat that is treated may still warp if the treatment is only superficial. Look for listings that specify the treatment process or that mention a specific standard, such as ThermoWood certification, which is a registered trademark from Finland. If the listing simply says 'heat treated,' ask the seller for details.

Avoid assuming that thermally modified wood is superior to a well-seasoned cedar or hemlock. Traditional sauna woods have been used for centuries, and many bathers prefer the natural aroma and appearance of untreated cedar. The choice depends on your preference for maintenance, look, and cost. Finnleo, a major sauna manufacturer, notes that 'wood choice is largely contingent upon user preference and style.'

When you read any technical claim in a sauna listing, check whether it has been verified by an independent lab. The ETL Listed Mark, for example, proves that a product has been independently tested and certified to applicable safety standards. That mark relates to electrical components, not wood, but it is a reminder to look for third-party validation of important claims.

Alternatives and other treatments

If you don't want thermally modified wood, you have other options. Naturally durable species like Western red cedar and northern white pine are common sauna woods. Cedar contains natural oils that resist moisture and insects, while pine is affordable but less durable. Some manufacturers use chemical treatments or coatings, but these can off-gas in high heat. Thermally modified wood avoids that because the treatment is heat-based and leaves no chemical residue.

For outdoor saunas, thermally modified wood is particularly useful because it handles moisture and temperature swings without needing a sealant. However, it is still wood, so it will gray over time if left exposed to sun and rain. You can find out more about exterior choices in our guide to outdoor saunas.

What to pick for your use

Questions

Is thermally modified wood the same as ThermoWood?

No. ThermoWood is a registered trademark from Finland with specific process standards. Thermally modified wood is a generic term that covers many heating methods. A listing that says 'thermowood' usually refers to the Finnish standard, but always confirm the details.

Does thermally modified wood have a smell?

It has a neutral, slightly smoky odor that fades quickly. It does not have the aromatic resins of cedar or pine. If you enjoy the scent of a traditional sauna, you may want to stick with an untreated softwood.

Can thermally modified wood be used outside a sauna?

Yes. The heat treatment makes it resistant to moisture, rot, and insects, so it works well for outdoor cabin walls and decks. However, it will still gray if left exposed to UV light, so most owners accept a silvered patina.

Does thermal modification make wood stronger?

It makes the wood more dimensionally stable and less prone to absorbing water, but the process can also make it more brittle. Manufacturers usually use thicker boards for structural parts to compensate.