Thermowood is timber heated to between 180 and 215 °C in steam, with very little oxygen. The heat changes the wood’s structure, so it takes up less water and moves less as the weather changes.
Heat-treated ash and pine reach durability classes that untreated softwood cannot, without chemical preservatives. The colour turns deep brown and weathers to silver-grey unless it is oiled.
Thermowood is slightly more brittle than untreated timber, so it is used for terraces, cladding and saunas rather than load-bearing frames.