Thermowood is manufactured using a method developed by VTT. The wood material is heated to a temperature of at leat 180 degrees while it is protected with steam. Besides providing protection, the steam also affects the chemical changes taking place in the wood. As a result of treatment, environmentally friendly Thermowood is created. Its color darkens, it is more stable than normal wood in conditions of changing humidity, and its thermal insulation properties are improved. If carried out at a sufficiently high temperature, treatment also makes the wood resistant to decay.