Heat-treated wood is ordinary wood that has been exposed to high temperatures (typically 160-240°C) under controlled conditions, without oxygen. This process permanently changes the wood’s chemical and physical structure, improving its durability, dimensional stability, and resistance to moisture. Heat treatment darkens the wood’s color and makes it a more stable material for many applications, especially in humid spaces like saunas.
What does heat-treated wood mean?
Heat-treated wood, also known as thermally modified wood, is wood material that has been processed at high temperatures in a controlled process. The treatment is carried out without oxygen, using steam as a protective agent. During the process, the wood is gradually heated to temperatures of 160-240°C for several hours, which causes permanent changes in the wood’s structure and properties.
Several chemical changes occur during heat treatment. The wood’s hemicellulose partially breaks down, which reduces the wood’s ability to bind water. Additionally, resin and other extractives are removed from the wood. These changes make the wood a more stable and durable material that doesn’t swell or shrink as much due to moisture effects as untreated wood.
In appearance, heat-treated wood is generally darker than untreated wood. The color varies according to the intensity of the heat treatment – the higher the temperature, the darker the result. The color change occurs throughout the wood’s structure, not just on the surface.
Why is wood heat-treated?
Wood is heat-treated primarily to improve its durability and performance characteristics. Heat treatment makes wood more dimensionally stable, meaning it reacts less to moisture fluctuations, which reduces wood movement, warping, and cracking. This is especially beneficial in spaces where humidity varies, such as saunas and bathrooms.
Another significant benefit is the improvement in wood’s biological durability. Heat treatment reduces wood’s susceptibility to mold and fungal growth, because the treatment changes the wood’s structure so that microbes no longer recognize it as food. This significantly extends the lifespan of wood products.
Heat treatment also improves wood’s thermal insulation properties. When the wood’s cellular structure changes during treatment, its thermal conductivity decreases, making it a better insulator. This is advantageous especially in sauna construction, where good thermal insulation is important.
Additionally, heat treatment is an environmentally friendly way to improve wood properties, as it uses no chemicals or toxins, only heat and steam.
How does heat-treated wood differ from ordinary wood?
Heat-treated wood differs from ordinary wood both in appearance and properties. The most visible difference is in color – heat treatment significantly darkens the wood’s tone, giving it a deep, warm color that extends throughout the entire wood. This makes it possible to work and sand the wood without the dark tone disappearing.
Moisture movement is considerably less in heat-treated wood. Untreated wood can move with moisture fluctuations by as much as 8-10%, while heat-treated wood typically moves only 3-5%. This makes it a more stable material that better maintains its shape.
Heat-treated wood is also lighter, as moisture and extractives are removed from the wood during treatment. Despite this, it is often harder on the surface than untreated wood, although internal strength may be slightly reduced during treatment.
In terms of service life, heat-treated wood is clearly longer-lasting than ordinary wood, especially in demanding conditions. Its biological durability is significantly better, which reduces maintenance needs and extends the material’s lifespan.
Which wood species are best suited for heat treatment?
Aspen and alder are excellent wood species for heat treatment, especially in sauna construction. Both are naturally low-knot and resin-free wood species, making them ideal for sauna bench boards. Heat treatment enhances the best properties of these wood species – it makes them even more durable in humid conditions.
Aspen is particularly popular in sauna construction because it doesn’t heat up excessively in the sauna and therefore feels pleasant against the skin. Heat treatment improves aspen’s dimensional stability and extends its service life, making it an excellent choice for benches and panels.
Alder, on the other hand, is known for its beautiful, reddish color, which deepens during heat treatment. It is soft and pleasant to the touch, making it well-suited for sauna benches.
Pine, such as radiata pine, is also well-suited for heat treatment. Heat treatment removes resin from pine and makes it more stable, which improves its usability in many applications, such as high-quality wood panels and boards interior panels and moldings.
What is heat-treated wood best suited for?
Heat-treated wood is excellently suited for sauna construction and humid spaces. Its better moisture movement control, durability, and natural resistance to mold make it an ideal material for benches, panels, and moldings. Additionally, heat treatment makes the wood’s surface smooth and pleasant, and it doesn’t heat up excessively in the sauna.
In bathrooms, heat-treated wood works well in panels, furniture, and surfaces. It withstands moisture better than ordinary wood, which extends product lifespan and reduces maintenance needs.
In interior use, heat-treated wood brings warmth and a natural feel even to modern spaces. It’s used in wall panels, interior moldings, and even furniture. The wood’s dark, warm tone suits many interior styles and creates a beautiful contrast with lighter materials.
In public spaces, heat-treated wood’s durability and stylish appearance make it a popular choice. It’s used, for example, in hotels and spas for impressive wood structures hotels, restaurants, and spas, where both aesthetic and functional properties are valued.
How should heat-treated wood be maintained?
Maintaining heat-treated wood is fairly simple, but a few basics are good to know. Clean surfaces regularly with mild soapy water and a soft cloth. Avoid strong detergents and abrasive materials that can damage the wood’s surface.
In saunas, heat-treated wood naturally darkens over time as a result of use. This is normal and doesn’t require special measures. If you want to slow the darkening, ensure proper ventilation of the sauna after use.
Although heat-treated wood is more durable than untreated wood, it too can get scratched. If the surface gets scratches, you can lightly sand them with fine sandpaper. Remember to sand in the direction of the wood grain.
Heat-treated wood generally doesn’t need surface treatment. However, if you want to enhance the wood’s natural beauty or protect it further, you can use wood oil that’s suitable for humid spaces. Oil enhances the wood’s deep color and protects the surface.
To avoid problems, don’t leave heat-treated wood in direct contact with water for extended periods. Although it withstands moisture better than ordinary wood, continuous water exposure can damage it over time.
We at Pieksäwood have over 20 years of experience in heat treatment, especially in treating aspen and alder. We manufacture all our products carefully in Pieksämäki, Finland, with a modern production line that ensures high quality. We want to offer you durable and beautiful solutions for saunas, homes, and public spaces. Contact us for expert guidance on your heat-treated wood needs.