The process
Wood, changed all the way through.
Thermal modification was developed in Finland and is governed by the International ThermoWood Association, whose handbook defines the process and the product classes. We're a member, and every board we sell is produced to that standard. Here's what actually happens in the kiln.
Three stages
What happens at 212 °C.
The whole treatment uses two ingredients: heat and steam. No preservatives go in, so nothing can leach out.
High-temperature drying
Kiln temperature climbs to around 100 °C and beyond, taking the wood down to near-zero moisture content. Steam shields the boards from checking.
Heat treatment at 212 °C
The core of the process. For our Thermo-D exterior boards, temperature holds at 212 °C for 2–3 hours. Hemicellulose — the sugar fungi feed on — permanently breaks down, and resin is driven off.
Cooling & conditioning
Water spray brings the temperature down and re-moistens the wood to a stable 4–7% — ready to mill, and far below where decay can start.
The numbers
Thermo-D, per the ITWA handbook.
These aren't our marketing claims — they're the published properties of the Thermo-D class, defined by the International ThermoWood Association and verified over roughly three decades of use in Nordic weather.
It's also why we can put a 25-year warranty against rot and decay behind every exterior board without flinching.
Thermo-D · exterior class
| Treatment temperature | 212 °C |
| Durability (EN 350) | Class 1–2 |
| Service life above ground | 25+ years |
| Equilibrium moisture | ≈50% lower |
| Swelling & shrinkage | ≈40–50% less |
| Chemicals added | None |
On site
What your installer should know.
Thermowood works like clear, dry pine — with four habits that keep the warranty intact.
Fasteners
Stainless only
The modified wood is slightly acidic, so standard galvanized fasteners can streak. Stainless steel screws and clips, every time.
Working it
Cuts like clear pine
Saw, rout, and sand it with ordinary tools — expect crisp edges and less gum on the blade. Seal cut ends to keep them matching the face.
Finish
Coat it — or don't
Left bare, exterior boards weather to an even silver-grey. A UV oil keeps the roast tone. Both are correct; it's a design decision, not maintenance.
Placement
Above ground use
Cladding, decking, ceilings, screens — yes. Ground contact or permanent immersion — no. That's the honest boundary of the class.
The fastest way to understand thermowood is to hold it. Order the sample box, or send your project for a price.