Thermally
Modified Timber

Thermally modified timber is natural wood that has been enhanced using heat and steam. The process permanently alters the internal structure of the timber, improving its behaviour in exterior environments without the use of chemical preservatives.

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What is Thermally Modified Timber

Thermal modification is a controlled process where timber is heated to high temperatures in a low-oxygen environment. The heat changes the chemical structure of the wood, reducing its ability to absorb moisture and improving its performance outdoors.

Improved durability
Greater dimensional stability
Reduced moisture absorption
No chemical treatment
Suitable for exterior use

Thermal Modification Process

Thermal modification is a controlled heat treatment that transforms responsibly sourced softwood into a more stable and durable material. Timber boards are placed in a specialised chamber where temperatures typically reach 180–230°C in a low-oxygen environment using heat and steam only.
During this process the chemical composition of the wood changes. Natural sugars are reduced and the cell structure becomes less able to absorb moisture from the surrounding environment. No chemical preservatives are added.

The result is timber with improved dimensional stability, enhanced durability and a deeper, more consistent colour throughout the section.

High temperature heat treatment
No chemical additives
Improved durability
Enhanced dimensional stability

What changes in the wood

Thermal modification works by altering the internal cell structure of the timber rather than coating or impregnating the wood with chemicals.

At high temperatures, hemicellulose within the cell walls breaks down and the structure of the wood changes. This reduces the ability of the timber to bind with water molecules. Because the wood absorbs less moisture, it becomes more dimensionally stable and more resistant to fungal decay. These microscopic structural changes are what give thermally modified timber its improved durability and long-term performance.

Reduced hygroscopic behaviour
Lower equilibrium moisture content
Improved resistance to decay
Greater long-term stability

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a visual of untreated wood versus thermowood

Untreated vs Thermally Modified Wood

Untreated timber naturally absorbs and releases moisture as environmental conditions change. This movement causes swelling, shrinkage and potential distortion over time.

Thermal modification alters the way wood interacts with moisture. By reducing the hygroscopic behaviour of the timber, the boards absorb significantly less moisture and remain more stable in service. This improved stability results in straighter boards, reduced movement and more predictable long-term performance in exterior applications.

Reduced moisture uptake
Less swelling and shrinkage
Improved dimensional stability
More consistent performance outdoors

typical timber cladding project for avara wood

Typical Applications

Thermally modified timber is widely used in exterior construction where greater stability and durability are required.

At Corell, thermally modified timber forms the basis of the Avara™ architectural timber cladding range. These materials are machined in Ireland and used across cladding, screening and exterior timber applications.

Timber cladding
Screening and fencing
Outdoor structures
Landscape elements
Architectural timber features

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Material Integrity

Thermally modified timber remains a solid, natural material. The process uses only heat and steam to improve durability and dimensional stability, without introducing additional materials or chemical modification.

In contrast, composite products are manufactured from a blend of plastic and wood fibre. While often selected for low maintenance, they behave differently in service and have limitations at end of life due to their mixed material composition.

As a natural timber product, Avara™ can be reused, recycled, or allowed to biodegrade, while maintaining the performance benefits of thermal modification.