Thermal modification of wood improves the weather resistance properties of wood. Due to diminished equilibrium moisture content, the moisture content of thermally modified wood when subjected to various weather conditions also diminishes. Other significant advantages imparted by heat treatment on wood regarding its use in external applications are in improving dimensional stability and in the consequent reduced stress caused by air temperature, moisture in wood, sunlight, and other physical properties.

Surface mould can also occur on the surface of Thermowood®. Due to the impurities spread by bacteria contained in air and by rain, mould colonies can form on untreated surfaces. However, these mould colonies are to be found only on the surface of wood and they do not weaken the wood structurally. Mould can be removed by washing, wiping or scraping.

In a similar manner, and as is the case with most natural materials, the UV radiation of sunlight makes thermally modified wood susceptible to colour changes. Direct exposure to sunlight causes colour changes with time in the original brown colour and at first leads to lightening of the colour and then turning grey within 2-3 years.

Five-year weather-stress trials have revealed that the moisture content of Thermowood® is about 50% lower than that of untreated wood. Surface-treating substances not containing pigment or those composed of thin coats of stain wear off thermally modified wood in the same way as they wear off untreated wood.

Thermowood® behaves better than unmodified wood as a surface to be painted. Exposure trials lasting 5 years have revealed that the functionality of vulcanised and water-dilutable acrylic paints was better on thermally modified boards than on untreated boards. Boards treated with these paints demonstrated less peeling off of the paint.

Exterior wall paints work well with Thermowood®. The best surface-treating systems for thermally modified wood consist of priming oil and solvent-dilutable alkyd paint or water-dilutable acryl finishing paint.

Research results

Moisture content during natural exposure, pine »
Colour change when exposed to weather »
Effect of treatment temperature on cracking and growth of surface mould »
Effect of substrate on peeling off of paint »