ThermoWood® is a wood material produced by using natural methods, heat and steam. Thermal modification improves the wood’s technical properties and ThermoWood is non-toxic, dimensionally stable, resistant to decay and resin free. It can be used inside or outdoors, in any climate.

ThermoWood® properties
  • Dimensional stability
  • Reduced equilibrium moisture content
  • Improved durability against decay
  • Reduced thermal conductivity
  • Resin removed
  • Consistent colour through the wood
  • Non toxic material
  • Reduced splitting strength
  • Slightly reduced bending strenght

Standard ThermoWood® treatment classification
There are two classes of heat treatment. Softwood and hardwood species have a separate classification since their properties clearly differ. Genuine ThermoWood has two standard treatment classes, Thermo-S and Thermo-D. 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.

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-S Softwood

  • building components
  • furnishing in dry conditions
  • fixtures in dry conditions
  • furniture
  • garden furniture
  • sauna benches
  • door and window components

Thermo-S Hardwood

  • furnishing
  • fixtures
  • furniture
  • flooring
  • sauna structures
  • garden furniture

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:

Thermo-D Softwood

  • cladding
  • outer doors
  • shutters
  • environmental constructions
  • sauna and bathroom furnishing
  • flooring
  • garden furniture

Thermo-D Hardwood

End use applications as in Thermo-S. If a darker colour is desired, Thermo-D should be used.

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

Softwoods (pine and spruce)

Thermo-S Thermo-D
Treatment temperature 190 °C 212 °C
Weather resistance + ++
Dimensional stability + ++
Bending strength no change -
Colour darkness + ++

Hardwoods (birch and aspen)

Thermo-S Thermo-D
Treatment temperature 185 °C 200 °C
Weather resistance no change +
Dimensional stability + +
Bending strength no change -
Colour darkness + ++

Unfortunately the unlawful use of word ThermoWood occurs in marketing of thermally modified timber. The technical data published by the International ThermoWood Association is not valid for all thermally modified timber that is available on the market. The data is valid only for genuine ThermoWood products that are produced under the independent quality control system audited by accredited notified body. Valid ThermoWood FC certificates are listed on auditor's website. ThermoWood product classification (classes Thermo-S and Thermo-D) is also copied by some nonmember companies. The authenticity of the product should always be confirmed.

The strength of the registered ThermoWood trademark is proven by the decision of the Office for Harmonization in The Internal Market (OHIM). In addition to EU, the ThermoWood trademark is registered also in Switzerland, Turkey, Japan, China, the USA, Canada, Russia and Iran.