ThermoWood® Explained: Accredited Timber for Guaranteed Performance and Sustainability
Written in collaboration with International ThermoWood Association
30th May 2025
Thermally modified timber species are fast becoming the go-to choice for sustainable, high-performance construction. ThermoWood® Ash and ThermoWood® Pine are two standout options that combine the beauty of natural timber with exceptional durability, thanks to the thermal modification process.
But ThermoWood® isn’t just a product, it’s a standard of quality and environmental responsibility. At Millworks we encourage specifiers and homeowners to look for the ThermoWood® trademark when sourcing thermally modified timber products, as a mark of authenticity and performance you can rely on.
What is ThermoWood® and how is it made?
ThermoWood® is a timber material produced by using a natural, chemical-free process involving only heat and steam. Developed in Finland, the thermal modification process heats timber to between 200-212°C in a controlled environment.
The thermal treatment permanently alters the cellular structure of the wood, reducing its moisture content and significantly improving its dimensional stability. This process reduces the risk of swelling, shrinking or warping; it also improves the timber’s resistance to fungal decay and insect damage, extending its service life without the need for additional treatment. As a result, ThermoWood® can be used across a wide range of external and internal applications, in any climate.
Certified quality, environmental responsibility
Third-party audited quality control is a key part of the ThermoWood® brand. As members of the International ThermoWood® Association, Millworks ensures that every piece of ThermoWood® we supply meets rigorous industry benchmarks.
When selecting thermally modified timber for your project, choosing registered ThermoWood® products from Millworks ensures you benefit from consistent quality and sustainable sourcing. All Millworks ThermoWood® products are treated to the Thermo-D classification, where “D” stands for durability. In accordance with EN 113 standard, its natural resistance to decay meets a minimum durability class 2 rating dependant on species.
Millworks ThermoWood® products are sourced from International ThermoWood® Association producers and carry FSC® or PEFC™ certification, ensuring complete transparency, traceability, and responsible forestry management.
Long-lasting finishes with factory coatings
Greater stability means less movement, making ThermoWood® products the ideal substrate for factory-applied finishes. Reduced timber movement means coatings remain intact and protective for extended periods, lowering ongoing maintenance requirements.
Factory-coated ThermoWood® products are particularly effective in demanding environments where durability and aesthetic retention are key, making them a reliable choice for both residential and commercial builds.
Designed for modern building systems
The low movement class of thermally modified timbers make them exceptionally well-suited to contemporary installation systems such as the secret clip system. This system comes with pre-fitted, precisely spaced clips, guaranteeing perfect alignment on every installation. Due to its increased stability, ThermoWood® performs very well on these systems, reducing the risk of movement-related issues such as cupping or uneven board alignment while preserving visual quality and long-term structural integrity.
Reliable performance
Whether you’re working with softwoods or hardwoods, ThermoWood® timber offers enhanced performance across the board. Our ThermoWood® Pine is a popular choice for both contemporary and traditional designs, providing a warm, characterful appearance with enhanced performance over standard softwoods. ThermoWood® Pine is sourced from slow-grown Nordic forests and undergoes thermal modification to elevate its durability class, making it suitable for exterior cladding and decking with minimal maintenance requirements.
For higher-spec designs or applications that require a high grade timber, ThermoWood® Ash delivers exceptional stability, rich colouration, and a Class 1 durability rating. Like ThermoWood® Pine, its thermal modification results in reduced movement and increased longevity of factory coatings for a premium finish.
Frequently asked questions
To learn more about our range of ThermoWood® products speak with our team on 01223 967733 or info@millworks.co.uk to discuss your project requirements.
