Wood has been a trusted building material for centuries, valued for its versatility, strength, and beauty. But in climates like the UK where moisture, temperature shifts, and biological decay challenge timber’s longevity, natural wood often needs chemical treatments or heavy maintenance to perform long-term. This is where thermally modified wood offers a durable and chemical-free solution for construction professionals.
What Is Thermally Modified Wood?
Thermally modified wood, also known as thermowood, is natural wood that’s undergone a high-heat treatment process, typically between 160–215 °C, in a controlled environment using only heat and steam. This alters the wood’s internal structure, reducing moisture absorption, increasing dimensional stability, and significantly improving resistance to rot, fungi, and insects. The result is a more durable, stable, and environmentally friendly material. All without chemicals.
Why It Works?
- Improved durability
Thermally modified wood withstands rain, humidity, and climate fluctuations far better than untreated timber. After thermal modification, softwoods like pine jump to durability class 2, while hardwoods like ash achieve a class 1 decay resistance of 25+ years. - Dimensional Stability
Thermal modification lowers the wood’s absorbency, meaning that exposure to moisture causes it to swell much less. It also improves dimensional stability – crucial for maintaining tight cladding lines, decking joints, or facade panels. - Low-Maintenance Performance
Exterior surfaces can be left untreated to weather naturally into a silvery-grey patina or maintained with periodic oiling to retain original color. Either way, thermally modified wood requires far less upkeep than conventional timber. - Eco-Friendly and Chemical-Free
No toxic preservatives are used. Thermal modification uses only heat and steam , making it safer for installers, occupants, and the environment.
Practical Considerations on Site
- Workability
Thermally modified wood has a lower weight and density, making the wood easier to work with. - Mechanical Properties
Thermal modification relieves internal stresses in the wood, resulting in straighter boards, less warping, and better dimensional stability. This makes it easier to machine, install, and maintain tight tolerances on site. - Less waste
Its predictable behavior simplifies planning for cladding, decking, and joinery applications. Thanks to meticulous sourcing, quality sorting, and end-matching profiles, you need to stock about 11% less material compared to non-thermally modified wood. - Finishing Options
Thermally modified wood’s rich, warm tones (from light gold to dark brown depending on species) create a premium appearance for facades, decks, and interiors. UV-protective oils can preserve colour or it can be painted to create a truly vivid aesthetic; otherwise, natural weathering produces a silvery grey finish. - Sustainability & Certification
Look for products with an Environmental Product Declaration (EPD) that provides transparent information about the environmental impact during its lifetime. Sourcing FSC or PEFC-certified stock supports responsible forestry standards.
Ideal Applications in Construction
| Application | Benefits |
| External Cladding | Dimensional stability maintains clean facade lines; excellent durability; contemporary aesthetic |
| Decking | Withstands wet conditions; resists fungal growth; barefoot-safe as it doesn’t overheat in the sun |
| Non-load-bearing outdoor structures (e.g. pergolas, fences, privacy screens, garden features) | Excellent dimensional stability and decay resistance; reduced warping and twisting over time; attractive appearance with minimal maintenance |
| Interior Spaces | Suitable for interiors, including humid spaces like saunas, pool areas, and changing rooms |
Why Specifiers Should Take Note
- Longer service life means reduced lifecycle costs
- No chemical preservatives or treatments required
- Excellent aesthetics for contemporary designs
- Sustainable sourcing supports green building certifications
- Predictable performance simplifies design calculations
About Thermory
Thermory is one of the world’s leading manufacturers of thermally modified wood, with over 25 years of expertise in the field. Combining advanced thermal modification technology with carefully sourced timber, Thermory supplies durable, stable, and beautiful timber solutions used in architectural and construction projects worldwide — from residential developments to large-scale commercial buildings.
