ThermoWood® vs. Thermally Modified Timber: Why the Difference Matters
- QTD Ltd
- Jun 4
- 4 min read
In the realm of wood enhancement, two prominent methods stand out: the patented ThermoWood® process and thermally modified timber (TMT). While both techniques involve heat treatment, the way they modify the wood’s structure—and their long-term performance—are significantly different in methodology, quality control, and the resulting product's performance. The TMT (High Heat Chamber) process has come on stream in recent years with some producers at a lower capital cost and is being marketed as performing the same process as the patented ThermoWood® process, which unfortunately is misleading customers who are not aware of the differences and believe they are getting ThermoWood®.
ThermoWood® Process: Open Cell, Precision and Consistency
Developed in Finland during the 1990s, the ThermoWood® process is a patented method that subjects timber to high temperatures ranging from 180°C to 230°C in a controlled steam environment. This treatment modifies the wood's chemical and physical structure, enhancing its durability, stability, and resistance to decay. The process is meticulously standardised, ensuring consistent quality across all batches. Each production cycle undergoes rigorous auditing three times annually by external parties, complemented by continuous internal quality assessments. For instance, ThermoWood® Association members are regularly audited by 3rd parties, testing ten samples from every production batch in their laboratories to ensure high standards are maintained.
One of the most critical distinctions between these two processes is whether the wood undergoes an open cell or closed cell transformation.
ThermoWood® is an Open-Cell Process that provides a more controlled approach to the wood's Relative Humidity, the surface heat across the wood, the internal heat through to the core, and the moisture content. The patented ThermoWood® process modifies the wood while keeping its cell structure open. This means the wood can still breathe, absorbing and releasing moisture naturally without warping or cracking. This also allows paints, stains, and adhesives to adhere properly, ensuring long-lasting coatings and finishes.

Image: ThermoWood® Yartek kilns
Thermally Modified Timber (TMT): Diverse Approaches
Thermally modified timber encompasses a variety of heat treatment methods aimed at enhancing wood properties. These processes generally involve heating the wood to temperatures between 180°C and 230°C, sometimes in the presence of steam or other inert atmospheres. However, the absence of a standardised procedure means that treatment parameters can vary widely, leading to inconsistencies in the final product's quality and performance. Without stringent quality control measures, the durability and stability of thermally modified timber can be unpredictable.
Thermally Modified Timber: Adopts a Closed Cell Process, which relies on pushing heat into the chamber and does not provide these controlled levels. In contrast, many thermally modified timber processes use a closed-cell approach, where the extreme heat collapses and seals the wood cells. While this initially makes the wood more water-resistant, it also makes it brittle and prone to surface cracking. The surface cracking appears after the timber has been exposed to the environment. Additionally, because the surface is sealed, coatings and finishes struggle to adhere properly, leading to peeling and premature failure.
Standardisation and Quality Control: The ThermoWood® process adheres to a patented, uniform method with regular external audits and internal quality checks, ensuring consistent product quality. In contrast, general thermal modification lacks a standardised approach, resulting in potential variability in product outcomes.
Process Parameters: ThermoWood® utilises specific temperature and steam conditions meticulously controlled throughout the treatment.
Other thermal modification methods employ different temperatures, durations, and environmental conditions, affecting the wood's final characteristics.
Product Performance: Due to its controlled process, ThermoWood® offers enhanced dimensional stability, reduced moisture content, and improved resistance to biological decay. The performance of thermally modified timber can vary based on the specific treatment applied and the absence of uniform quality assurance, which can ultimately lead to the timber’s premature failure in performance.
Key Advantages of ThermoWood® over TMT
✅ Better Dimensional Stability – The open cell structure prevents warping and cracking, making ThermoWood® ideal for both interior and exterior use.
✅ Stronger, More Durable Wood – ThermoWood® retains structural integrity, while closed-cell thermally modified timber is more prone to brittleness.
✅ Superior Coating & Adhesion – Paints, stains, and adhesives bond effectively to ThermoWood®, while thermally modified timber often struggles with adhesion.
✅ Proven, Patented Process – ThermoWood® is backed by decades of research, ensuring reliable and consistent results. General TMT processes vary widely and lack the same level of quality control.
While both ThermoWood® and thermally modified timber improve wood properties through heat treatment, ThermoWood® is far superior in every respect. Its open cell structure, strict quality control, and patented process result in a stronger, more stable, and longer-lasting wood product. In contrast, thermally modified timber's closed-cell approach often leads to brittleness, poor coating adhesion, and inconsistent quality.
For those seeking the best in modified wood, ThermoWood® is the clear choice—delivering unmatched durability, stability, and performance for years to come.
Currently, there are no ThermoWood® Kilns in the UK as the main processors are based in Europe, as you can see on the ThermoWood® association members' website https://www.thermowood.fi/
One TMT kiln became operational a few years ago in the UK, which is not a ThermoWood® Jartek Kiln.
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