Thermowood is timber that has been transformed using nothing but heat and steam. No chemicals, no preservatives - just controlled high temperature that permanently changes the wood's structure, making it more stable, more durable and far less prone to rot. For an industry under pressure to move away from chemical treatments and tropical hardwoods, it is one of the most important materials of the decade.
How thermal modification works
The process heats timber to between roughly 160°C and 230°C in a low-oxygen environment, usually under steam to prevent the wood from burning. At these temperatures the chemistry of the wood itself changes. The hemicelluloses - the sugar-rich components that readily absorb water and feed fungi - break down. The result is a piece of wood that behaves very differently from the board that went into the kiln.
Three things happen that matter to anyone specifying timber:
- It absorbs far less water. With the hemicelluloses degraded, the wood's equilibrium moisture content drops dramatically. It swells and shrinks much less with changing humidity.
- It becomes durable. Decay fungi can no longer find the food source they need, so even non-durable species like pine and ash gain serious rot resistance.
- It darkens. Thermal modification gives pale woods a rich, uniform brown tone right through the board - not just a surface stain.
Thermal modification reduces the wood's bending strength and can make it slightly more brittle. Thermowood is excellent for cladding, decking and joinery, but it is not a structural replacement for untreated timber in load-bearing applications.
Why the industry is switching to it
For decades, outdoor and wet-area timber meant one of two things: tropical hardwoods like teak and ipe, or softwoods pumped full of chemical preservatives. Both are now problematic. Tropical hardwoods raise sustainability and legality concerns and cost a fortune. Chemical treatments face tightening regulation, especially across Europe and North America.
Thermowood sidesteps both. It takes fast-growing, sustainable, locally available species - pine, spruce, ash, poplar - and upgrades them into a premium material using only heat. The environmental story is clean, and the performance genuinely competes with tropical hardwood for many uses.
Where thermowood excels
| Application | Why thermowood works |
|---|---|
| Exterior cladding | Dimensional stability stops cupping and gapping; durability resists weather |
| Decking | Low movement and rot resistance in the toughest wet-and-dry cycling |
| Saunas & wet rooms | Stays stable in heat and humidity; the classic original use |
| Window & door joinery | Minimal swelling keeps moving parts working year-round |
| Garden & landscape | Long service life without ground-contact chemical treatment |
Understanding the modification classes
Not all thermowood is equal. The treatment intensity - mainly the peak temperature - determines the balance between durability and strength. Producers usually offer two grades:
- Thermo-S (Stability): treated at lower temperatures (around 190°C). Optimised for dimensional stability with less strength loss. Good for interior and semi-exposed joinery.
- Thermo-D (Durability): treated hotter (around 212°C and up). Optimised for maximum decay resistance and outdoor service. The grade for cladding and decking.
Knowing which class you are buying matters: a Thermo-S board used as exterior decking will not last like a Thermo-D board, and a Thermo-D board has given up more strength than you may want for some joinery.
Caring for thermowood
Thermowood is durable, but the rich brown colour will silver over time under UV just like any wood unless it is finished. If you want to keep the dark tone, apply a UV-protective oil and re-coat periodically. If you are happy with a natural silver-grey patina, you can leave it untreated and it will still resist decay. Either way, the structural durability is built in - the finish is purely about appearance.
The bottom line
Thermowood delivers much of the stability and durability of tropical hardwood, from sustainable species, with a clean environmental story and no chemical treatment. It is not a structural timber and it costs more than untreated softwood, but for cladding, decking, joinery and wet areas it is often the smartest specification available today. If you are weighing it against teak or untreated wood, the honest way to decide is to compare lifetime cost - not just the price per board.