If I came across this home in the middle of the woods, I’d be a little scared. It definitely doesn’t look like a place where I want to live with its high grey walls. So why feature it? Because it’s clad entirely in so-called ThermoWood which is made it Finland. ThermoWood is basically sustainably harvested timber, which undergoes a three-stage heat treatment that eliminates the need for any sort of chemical treatment to be applied to it. The home picture above was made by the Polish architecture firm też architekci.


ThermoWood can be made from both hardwoods and softwoods, and the heat treatment applied to it is said to improve the dimensional stability and biological durability of the wood. In the first step of the treatment process, heat and steam are used to raise the kiln temperature to 130°C, which reduces the moisture content of the wood to nearly zero. The second step is the thermal modification phase, and here the temperature is raised to between 185-215°C and remains constant for 2-3 hours. The third and last step is the cooling and conditioning phase, and involves lowering the kiln temperature by using water sprays. Once the temperature reaches 80-90°C re-moisturizing of the wood takes place to bring its moisture level back to 4-7%.

teh process

This treatment process also reduces the thermal conductivity of the wood and thereby improves its insulation properties. And because of the high temperatures used to treat the wood, all resin is removed from it.

While a lot of energy is used to treat the wood, it is, according to the manufacturers, still quite low. Needless to say, the use of this wood could go a long way towards reducing the use of chemically treated wood. This heat treatment process works on many different kinds of wood, including pine and spruce, which are widely available and grow fast. As such it makes a great material for external cladding.


They have also performed standard tests on ThermoWood (EN 113, ENV 807) in a lab, and these tests proved that the wood has a much improved biological durability as compared to chemically treated wood. This durability is the result of removing natural food sources from the wood and changing its chemical and structural composition. Also, the higher the temperature of the treatment process, the more the wood is resistant to fungus.

ThermoWood has a very unique dark “walnut color” which is quite homogenous from board to board. By further treating them with oil based stain products, the boards can also be effectively protected against UV rays. If this is not done, the boards will turn an even charcoal grey over time.

Needless to say, using this type of wood is a very eco-friendly and green choice.