Thermowood is timber that has been made durable by heating to 180-220°C, without the use of chemicals. The high temperature changes the structure of the wood: it absorbs less moisture, is more resistant to rot, and becomes dimensionally stable. The result is a durable, environmentally friendly product with a warm brown colour.
How is Thermowood made?
The process (thermal modification) consists of three phases:
1. Drying — The wood is slowly heated to 100°C, moisture evaporates
2. Heating — The temperature is raised to 180-220°C. The wood sugars and resins change chemically.
3. Cooling and stabilising — The wood slowly cools and is reconditioned with steam
The entire process takes 2-3 days. The higher the temperature, the more durable the wood — but also the more brittle.
Properties
| Property | Thermowood | Untreated spruce |
|---|---|---|
| Durability | Class I-II (20+ years outdoors) | Class IV-V (2-5 years) |
| Moisture absorption | 40-50% less | Normal |
| Movement (shrink/swell) | Greatly reduced | Normal |
| Colour | Warm brown (weathers grey outdoors) | Light yellow |
| Hardness | Slightly lower | Normal |
| Weight | Lighter (from drying) | Normal |
Where is Thermowood used?
- Facade cladding — Popular choice for a warm appearance without maintenance
- Decking — Dimensionally stable, less prone to splintering
- Sauna lining — Low thermal conductivity (doesn’t burn your skin)
- Garden furniture — Attractive and durable
- Window and door frames — Less movement than untreated wood
Thermowood vs. other options
| Thermowood | Pressure-treated | Douglas fir | Hardwood | |
|---|---|---|---|---|
| Chemicals | No | Yes | No | No |
| Durability | 20+ years | 15-20 years | 15-20 years | 25+ years |
| Price per m² | Medium-high | Low | Medium | High |
| Appearance | Warm brown | Greenish | Red-brown | Variable |
| Environment | Good | Moderate | Good | Check origin |
| Maintenance | Minimal | Minimal | Minimal | Minimal |
Points to note
- More brittle than untreated wood — Always pre-drill when screwing. Thermowood can split when screwing without pre-drilling.
- Greying — Outdoors, Thermowood weathers to a silver-grey, just like Douglas fir. Oil can maintain the brown colour (oil 1-2x per year).
- Not structural — Thermowood is slightly less strong than untreated wood. Use it for cladding and visible surfaces, not for load-bearing structures.
- Check temperature class — Thermo-D (210-220°C) is more durable than Thermo-S (180-190°C).
Related terms
- Pressure-treated wood
- Douglas fir
- Hardwood
- Softwood
- Weatherboard cladding
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