What Is Thermally Modified Wood in a Cabin?

What Is Thermally Modified Wood?

Thermally modified wood is a type of wood that has been heat-treated in a controlled process. The treatment changes the wood's chemical structure, giving it a darker color and making it more resistant to moisture and decay. The process is sometimes called thermowood, and it is used in a variety of exterior and interior applications, including sauna cabins.

In a sauna, thermally modified wood is used for wall panels, benches, and sometimes the exterior of the cabin. Because the treatment reduces the wood's ability to absorb water, it is less likely to warp or swell in the humid conditions of a traditional sauna. The wood also becomes less prone to cracking. For a deeper look at how sauna interiors are built, see our guide on solid wood vs. engineered panels.

The darker, uniform color of thermally modified wood is a result of the heat treatment. Makers often market this as a modern, Scandinavian look that does not require staining or oiling to achieve a consistent tone.

Why Use Thermally Modified Wood in a Sauna?

Saunas are high-heat, low-humidity environments. Traditional saunas, which are heated by wood or electric stoves, typically have humidity below 20%, according to the Cleveland Clinic. Thermally modified wood is also used in steam rooms and wet saunas because it sheds water easily and resists swelling.

Thermally modified wood offers several advantages for sauna builders. Its low moisture absorption means it will not swell as much as untreated softwoods when you splash water on the rocks. It is also more dimensionally stable, so it is less likely to cup or twist over time. Makers claim that thermowood requires less maintenance than cedar or hemlock, because it does not need periodic oiling to preserve its appearance.

Another benefit is the low resin content. Some wood species, like pine, can exude resin when heated, which can stain the wood and produce a strong smell. Thermally modified wood has much of the resin removed during the heat treatment, so it is often chosen for sauna interiors where resin could be a problem. If you are comparing wood types, our cedar vs. hemlock comparison covers two common alternatives.

What Do Sauna Makers Claim About Thermowood?

Sauna manufacturers that offer thermally modified wood say it is a premium material. They claim it lasts longer than conventional sauna wood, resists decay and insect damage, and needs less refinishing. Some also say that the darker, even tone of thermowood gives a sauna a contemporary look that fits both indoor and outdoor settings.

It is important to treat these claims as marketing unless they are backed by product specifications. Ask the maker for the specific heat-treatment process, the wood species used, and any warranty details. Finnleo, a long-time sauna maker, notes that wood choice is largely a matter of user preference and style, so you should consider how the wood feels and looks in person.

Thermowood is not a single product. Different manufacturers use different base woods and different treatment schedules, so the properties can vary. Check whether the maker uses the treated wood only in the interior or also for structural parts like benches and supports.

Considerations and Trade-Offs

Thermally modified wood is not without its trade-offs. The heat treatment can make the wood more brittle, so it may not be the best choice for areas that see heavy impact, such as floor slats in a high-traffic sauna. Ask the maker how the wood performs under loading and whether they use a thicker plank for benches.

The appearance is also different. If you want the classic smell and color of cedar, thermowood will not provide that. Cedar has a distinct scent that many sauna lovers enjoy, while thermally modified wood is more neutral. For a fuller discussion of which sauna wood is best, see our article on which sauna wood is best.

Finally, consider how the wood is finished. Some thermally modified wood is left unfinished, while others have a surface treatment. In a sauna, finishes must be able to withstand repeated heating and cooling. If you are unsure, ask the maker whether the wood is sealed and what kind of maintenance is expected.

How to Choose a Thermowood Sauna

If you are thinking about a sauna with thermally modified wood, start by looking at the overall construction. A good sauna cabin should use solid wood panels, not engineered composites, and the wood should be properly dried. For electrical installations, look for a certification mark such as ETL listed, which indicates that the product has been independently tested to safety standards.

Consider the climate where you will place the sauna. Thermal modification improves moisture resistance, which is especially valuable for outdoor saunas that face rain and snow. If you live in a humid area, thermowood can be a smart choice. For a traditional Finnish sauna, you might still prefer cedar or hemlock for their appearance and durability. Our guide to best cedar saunas can help you compare options.

What to pick for your use

Questions

Is thermally modified wood good for saunas?

Yes, many sauna makers use thermally modified wood because it resists moisture, resists swelling, and needs little upkeep. The treatment also reduces resin content, which helps keep the interior clean. However, the wood can be more brittle, so ask about plank thickness and how it is used for benches.

Does thermowood need oiling?

Generally, thermally modified wood is sold without a finish, and makers claim it does not need oiling to maintain its color or moisture resistance. Still, some makers apply a protective surface treatment. Always follow the manufacturer's care instructions, since sauna heat and weekly use will test any finish.

Is thermowood more durable than cedar?

Makers often claim that thermally modified wood resists moisture and decay better than cedar, and that it is less likely to warp. Cedar, however, has natural oils that give it a pleasant scent and its own longevity. Durability also depends on the thickness of the wood and how the sauna is built.

Does thermally modified wood smell like cedar?

No. Thermally modified wood has a neutral smell, because the heat treatment removes many of the volatile compounds found in fresh wood. If you enjoy the aroma of cedar in a traditional sauna, you may miss that with thermowood.

Can I use thermowood in an outdoor sauna?

Yes. Thermal modification improves the wood's resistance to moisture, which is particularly useful for outdoor saunas that face rain, snow, and temperature swings. Many outdoor sauna makers offer thermowood as a cladding or interior option. Check that the wood is appropriately treated for your climate.