ThermoWood®

ThermoWood®

ThermoWood® is a registered trademark owned by the ITWA. Industrial scale ThermoWood® process was developed in cooperation with VTT, Technical Research Centre of Finland and wood industry companies. In 1994 the process was patented and word thermowood was registered as commercial ThermoWood® trademark by VTT.

The license agreement on the exploitation of the patent was concluded between VTT and the ITWA in 2001. By the same agreement, ownership of the ThermoWood® trademark was transferred to the ITWA. Therefore, the word thermowood is not a generic name for thermally modified timber. It should be noted that CEN/TS 15679:2007 Thermal Modified Timber - Definitions and Characteristics uses the generic name Thermally Modified Timber.

 

ThermoWood® PROCESS

The ThermoWood® process is an industrial scale thermal modification method in which wood is modified at elevated temperature in the presence of steam under atmospheric pressure, without added chemicals. The process permanently changes wood structure and moisture behaviour, resulting in reduced equilibrium moisture content, improved dimensional stability, and enhanced resistance to biological degradation. The result is a natural wood material with permanently improved properties, suitable for both indoor and outdoor use, in all climates.

THERMO-S AND THERMO-D ThermoWood® PRODUCT CLASSES

ThermoWood® products are classified into two standard treatment classes, designed to meet different performance requirements.

Thermo-S (Stability)

Thermo-S is characterised by enhanced dimensional stability and refined appearance. It is suitable for applications where visual quality and stability are key.

Typical applications include:

  • Interior cladding and ceilings
  • Furniture and fixtures
  • Sauna interiors and benches
  • Exterior elements in limited weather exposure

Thermo-D (Durability)

Thermo-D offers higher biological durability and weather resistance, making it ideal for demanding outdoor conditions.

Typical applications include:

  • Exterior cladding
  • Decking and terraces
  • Garden structures and outdoor furniture
  • Doors, windows and other exposed elements

ThermoWood® products are only made from the wood species as shown in the table below because scientic studies have shown them to meet the quality requirementis set for the finished product. 

CERTIFIED QUALITY YOU CAN TRUST

ThermoWood® is based on a patented thermal modification process developed through extensive research. Production is monitored through an audited quality control system, with regular inspections by independent third parties.

The ThermoWood® trademark guarantees that:

  • the production process follows defined technical criteria
  • raw materials originate from responsibly managed, certified forests
  • product properties are consistent and verified

This distinguishes genuine ThermoWood® from unverified or imitation thermally modified wood products.

 

SUSTAINABILITY AND LIFECYCLE

ThermoWood® directly supports sustainable construction and long-term resource efficiency.
It is made from renewable raw material, without chemical additives, and offers a long service life with minimal maintenance requirements.

At the end of its lifecycle, ThermoWood® can be reused, recycled or utilised for energy in the same way as untreated wood. Its low emissions make it a safe choice for both indoor environments and outdoor applications. Read more about sustainability here.

 

WHY CHOOSE ThermoWood®

ThermoWood® offers a unique combination of technical performance, aesthetics and environmental responsibility.

Key benefits include:

  • Excellent dimensional stability with reduced swelling and shrinkage
  • Improved resistance to decay and moisture
  • Resin-free material with consistent colour throughout the wood
  • Reduced thermal conductivity, ideal for cladding, saunas and outdoor structures
  • Non-toxic and safe for indoor use, with low VOC emissions
  • Suitable for use in varying and demanding climate conditions

ThermoWood® products are not intended for load-bearing structures, but they excel in architectural, decorative and functional applications where durability and appearance matter.

 

LEARN MORE

Detailed technical information, installation guidelines and performance data are available in the ThermoWood® Handbook 2025, available for download here.

For product-specific guidance, we recommend contacting ThermoWood® manufacturers directly.

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International ThermoWood Association / Lämpöpuuyhdistys ry

Siltasaarenkatu 12 A
00530 Helsinki
FINLAND

 

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