Standard ThermoWood® treatment classification

Softwood and hardwood species have a separate classification since their properties clearly differ. There are two classes of heat treatment. Having more than two classes is not reasonable since wood properties change slowly at first as the temperature increases. Once the treatment temperature exceeds 200 °C, the properties change rapidly. Using more than two classes would generate a risk of mixing properties of different classes. 215 °C is sufficient as a maximum temperature value yet is not so high that the effects of heat treatment on the wood's structural properties would be significant.
In the standard class of ThermoWood treatment, swelling and shrinkage of wood due to moisture, colour change, and biological durability are emphasised as key properties.

Since the ThermoWood material to be supplied to industrial customers is heat?treated in accordance with the agreements between the purchaser and producer, the processing level can be carefully optimised according to the end use application. In this case, the material will be ThermoWood that is not categorised according to the standard treatment classification scheme.


Genuine ThermoWood has two standard treatment classes, Thermo-S and Thermo-D.

Thermo-S

The letter 'S' in 'Thermo-S' stands for 'stability'. Along with appearance, stability is a key property in the end use applications of the products in this treatment class. The average tangential swelling and shrinkage due to moisture for Thermo-S class treated wood is 6-8%. Thermo-S class ThermoWood is classified as relatively durable according to the standard EN 113; i.e., its natural resistance to decay meets class 3 requirements.

Recommended end use applications for Thermo-S class heat-treated timber:

Thermo-D

The letter 'D' in 'Thermo-D' stands for 'durability'. Along with appearance, biological durability is a key property in the end use applications of products in this treatment class. The average tangential swelling and shrinkage due to moisture for Thermo-D class treated wood is 5-6%. Thermo-D class ThermoWood is classified as durable according to the EN 113 standard; i.e., its natural resistance to decay meets class 2 requirements.

Recommended end use applications for Thermo-D class heat-treated timber:

Summary of the effects of the ThermoWood process on wood properties, by treatment class

Softwoods (pine and spruce)

Hardwoods (birch and aspen)