What is thermally modified wood?
The Wood Guide · Calculating with wood
TreatmentThermally modified wood
Wood made more durable with nothing but heat and steam. No biocides, no heavy chemicals — but there is a downside you need to know about.
- Thermowood method
- Thermo-S 180 °C · Thermo-D 212 °C
- Without chemicals
Thermally modified wood
Wood made more durable with nothing but heat and steam. No biocides, no heavy chemicals — but there is a downside you need to know about.
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Thermally modified wood in brief
Also called Thermowood: a way of making wood more durable with nothing but heat and steam.
The idea is old — German researchers were already experimenting with it in the 1930s — but it only became practical in the 1990s, when the Finnish research centre VTT perfected the Thermowood method. Today the International Thermowood Association oversees the standard, which ensures consistent quality.
What happens: by heating the wood to well above a hundred degrees, the hemicellulose breaks down. That is precisely the component fungi live on and that holds moisture in the wood. Less hemicellulose therefore means both less rot and less movement.
Thermo-S is heated to 180 °C and is intended for indoors; Thermo-D goes up to 212 °C and is suitable for outdoors. That letter on the label determines where the board belongs.
- Method
- Thermowood, perfected by VTT (Finland)
- Process
- drying, modification, reconditioning
- Temperature
- 180 °C (Thermo-S) or 212 °C (Thermo-D)
- Duration
- 2 to 3 hours at temperature
- Final moisture content
- 4 to 6%
- Chemicals
- none
Fair is fair: the heating also makes the wood more brittle and somewhat less strong. That makes thermally modified wood less suitable for load-bearing structural parts — it is a cladding and decking timber.
What happens in the kiln
- Drying to almost zeroThe wood is first heated to 130 °C to remove virtually all of the moisture.
- Thermal modificationThen it goes to 180 °C (Thermo-S) or 212 °C (Thermo-D), and stays there for two to three hours. This is where the hemicellulose breaks down.
- Cooling and reconditioningControlled cooling and bringing the moisture content back up to 4–6%, so the wood does not come out of the kiln brittle.
The first research dates from the 1930s (Stamm and Hansen), followed by White in the 1940s and Bavendam, Runkel and Buro in the 1950s. It was not until the 1990s that VTT turned it into an industrial process.
What it gives you, and what it costs
What you gain
- Better stability: less sensitive to changes in moisture and temperature
- Higher durability through the breakdown of hemicellulose
- No chemicals — not even when disposed of or burned
- A warm, deeper brown colour right through the wood
- Fast-growing European softwood becomes usable outdoors
What it costs
- The wood becomes more brittle and somewhat less strong
- Less suitable for load-bearing structural parts
- Outdoors, the colour still fades to grey without UV protection
- More expensive than ordinary pressure-treated softwood
What thermally modified wood is used for
Cladding
Its strongest use: stable, attractive in colour and free of chemicals.
Decking
Thermo-D is suitable for outdoors. See also our guide to wood for your decking.
Garden furniture
Turns up as a low-maintenance alternative, although it is more brittle than hardwood.
Saunas and interior cladding
Thermo-S: the wood moves little and has a pleasant roasted smell.
In performance, thermally modified wood sits between pressure-treated softwood and Accoya: more durable than the former, less so than the latter, and without the chemistry of either. The full comparison is in the Accoya guide.
Frequently asked questions about thermally modified wood
Is thermally modified wood as durable as tropical hardwood?
Not quite, but it comes close: Thermo-D usually reaches class 2 to 3, compared with class 1 for teak. The big advantage is that no chemicals or tropical forest are involved.
What is the difference between Thermo-S and Thermo-D?
The temperature. Thermo-S is heated to 180 °C and is intended for indoor uses; Thermo-D goes up to 212 °C and is suitable for outdoors. Look for that letter on the label.
Why is thermally modified wood more brittle?
Heating breaks down hemicellulose — exactly what makes the wood more durable also makes it less tough. That is why it is suitable for cladding and decking boards, but not for load-bearing beams.
Does the brown colour last?
Indoors, yes. Outdoors it fades to grey under UV light, just like any other untreated wood. If you want to keep the colour, treat it with an oil or stain with a UV filter.
Thermally modified wood in perspective
One of the ways to make ordinary wood more durable — without chemicals, with a downside.
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Photos via Wikimedia Commons: Fraxinus — Anubis100, CC BY-SA 3.0 · Picea abies — Miladin, CC BY-SA 2.5 si