What's the Difference Between ThermoWood® and Accoya®?

QTD Ltd
13 minutes ago
3 min read

Explore the Benefits of Modified Timber for UK Cladding Projects

You Ask – We Answer

"What's the difference between ThermoWood® and Accoya®?"

When choosing timber cladding for a building project, understanding your options can help you make a long-term, cost-effective, and environmentally conscious decision. Two of the most advanced modified timber products available in the UK today are ThermoWood® and Accoya®. While both offer superior performance over untreated timber, the differences in their processing methods, properties, and cost make them suitable for different types of projects.

Let’s explore the details.

What Are ThermoWood® and Accoya®?

ThermoWood® is produced through a high-heat and steam process that alters the structure of the wood at a cellular level. This thermal modification enhances stability, durability, and resistance to rot—without using chemicals. ThermoWood® is available in both softwood and hardwood varieties, including:

Scandinavian Pine
New Zealand Radiata Pine
American Ash, Tulip/Poplar
African species such as Ayous, Frake, Movingui, and Iroko

Accoya® is made from fast-growing Radiata Pine that undergoes a process called acetylation. This chemical modification changes the wood at the molecular level to dramatically improve dimensional stability and resistance to decay. The result is a timber product with Class 1 durability and a long lifespan.

Comparison Table: ThermoWood® vs Accoya®

Feature	ThermoWood®	Accoya®
Timber Species	Various softwoods & hardwoods	Radiata Pine
Modification Process	Thermal (Heat + Steam)	Acetylation (Chemical)
Durability Class	Class 1–2	Class 1
Dimensional Stability	Excellent	Exceptional
Lifespan Above Ground	25–35+ years	50+ years
Resistance to Decay	High	Very High
Chemical Use	None	Acetic anhydride (non-toxic)
Colour	Honey-brown, ages to grey	Pale cream, ages to grey
Cost	More affordable	Higher investment
Sustainability	FSC® / PEFC certified	FSC® certified

Which Is Best for Your Project?

ThermoWood® is ideal for:

Residential and commercial cladding
Outdoor structures
Projects seeking improved performance at a cost-effective price
A natural, rich colour that can weather or be finished

Accoya® is suitable for:

High-end applications
Areas with high moisture exposure
Window frames and doors
Long-term durability with minimal maintenance

Environmental Benefits

Both options are considered sustainable, but in different ways:

ThermoWood® uses a chemical-free process and makes use of timber species that would not usually perform well externally. This reduces reliance on tropical hardwoods, helping to preserve rainforest ecosystems.
Accoya® modifies fast-growing, responsibly sourced Radiata Pine into a durable timber alternative, significantly extending its useful life and reducing replacement frequency.

Real-World Perspective

While Accoya® boasts impressive warranties and a 50+ year service life, consider the practical reality: will you still own the property in 35 years? ThermoWood® offers more than enough longevity for most projects, and at a lower cost.

Additionally, the higher cost of Accoya®—despite being made from a relatively inexpensive timber—often reflects the complexity of the process and brand positioning, rather than a dramatic increase in practical value for typical cladding applications.

Both ThermoWood® and Accoya® are excellent choices for external cladding in the UK, offering far superior performance to traditional timber. Your decision should come down to your budget, design goals, and how much durability is truly necessary.

Choose ThermoWood® if you want an affordable, beautiful, and high-performing natural cladding solution with proven longevity.
Choose Accoya® for ultra-high performance, extreme dimensional stability, and low maintenance—particularly in exposed environments or if used for windows and doors.