When Designboom described this house in Poland, designed by też architekci, as “a stealthy and brooding presence” I thought that was for sure, along with some other adjectives. But the next sentence caught my eye: “the dwelling is entirely clad with darkthermowood.” What’s that?

ThermoWood is a Finnish product, sustainably harvested timber that is subjected to a three-stage heat treatment. According to their website, it is made from both hardwoods and softwoods, which both benefit:

The process improves dimensional stability and biological durability of wood. Another improvement is in the insulation properties of the final material, the process leads to a reduction in thermal conductivity. Due to the high treatment temperatures the resin is removed from the wood.

It could go a long way to reducing the need for chemically treated wood, and create a bigger supply of wood that could be used for exterior uses, since the treatment works on soft woods like pine and spruce that are fast-growing and common. Although it uses more energy to go through the heat treatment process, according to their analysis it is still pretty low.

Standard tests (EN 113, ENV 807) made in laboratory conditions have proven a significant improvement in biological durability. Improvements in biological durability are a result of the removal of natural food sources in the wood and also changes in the chemical and structural composition. Levels of resistance to fungal decay increase as higher temperatures are used.

I could only find one North American supplier, Novawood, which appears to make Thermowood out of ash.

An added benefit to the Thermowood process is the unique “walnut colour” of the wood becoming homogeneous throughout the boards. It’s possible to protect this rich colour against UV rays by using oil based stain products. Otherwise, the colour of the wood will patina and turn an even grey colour overtime. This “greying” does not have an effect on the natural durability of the material.

I had not heard of ThermoWood before, but in fact versions of it are pretty common in North America, under the description of Thermally Modified Wood. The ThermoWood people back in Finland complain that “unfortunately the unlawful use of word ThermoWood occurs in marketing of thermally modified timber.” This Duluth company,Arbor Wood, appears to make a pretty similar product, also out of ash.

Another interesting alternative to chemically treated wood!