In construction in Spain, wood for outdoor use faces a constant challenge: humidity, saltpetre and sudden changes in temperature. Conventional wood tends to warp, crack or deteriorate quickly.
Thermally modified wood is a natural and sustainable solution. The thermo-d process modifies its structure using heat and steam, without adding chemicals. The result is a more stable, durable material with improved moisture resistance.
WHAT IS TERMO-D (THERMALLY TREATED WOOD)
Heat treatment is carried out in closed kilns where the wood is exposed to high temperatures in a controlled atmosphere without free oxygen. Only steam is used as protection.
There are two main levels:
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- Termo-s (~180 °C): a gentler treatment, designed for interiors. Provides stability, but not sufficient for exteriors.
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- Termo-d (~212 °C): a more intense treatment, designed for outdoor use. Improves dimensional stability and biological resistance.
The cycle can last between 24 and 72 hours. During this time, the heat breaks down sugars and resins that normally feed fungi. It also reduces the wood's ability to absorb water. As a result, thermo-treated wood can withstand humid environments or variable climates where untreated wood would quickly warp or deteriorate.
THE STEP-BY-STEP PROCESS
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- Controlled heating: the temperature is gradually raised to around 212 °C in an oxygen-free atmosphere to prevent combustion.
- Internal transformation: heat and steam change the composition of the wood, removing resins and sugars. As a result, it absorbs less water and retains its shape better.
- Cooling and stabilisation: the wood regains a balanced moisture level, ensuring more consistent behaviour in response to climate changes.
- Optional finish: water-based oil, with or without pigment, can be applied to enhance colour and protect the appearance.
ADDITIONAL BENEFITS
Thermo-d wood offers a number of practical advantages:
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- Moisture resistance: being less hygroscopic, it absorbs less water. This means less risk of deformation, even in coastal areas or areas with changing climates.
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- Greater thermal comfort: Technical studies have shown that heat-treated wood reduces its thermal conductivity., helping to improve the insulation of façades and enclosures (Sandberg et al., 2013 – Journal of Wood Science).
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- Natural protection against fungi: by eliminating the sugars that feed them, it becomes a material less susceptible to rot (Militz & Altgen, 2014 – Wood Material Science & Engineering).
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- Real sustainability: life cycle assessment (LCA) show that, over a 30-year period, thermally treated wood has a lower environmental impact than conventional wood. The reason is simple: it requires fewer chemicals and less maintenance throughout its useful life (Kutnar & Burnard, 2014 – BioResources).
As a technical note: in very intense processes, wood can become somewhat more fragile. Therefore, the choice of species and level of treatment must always be adapted to the end use.
THERMO-D + YAKISUGI: AN EXTREMELY DURABLE COMBINATION
Yakisugi is an ancient Japanese technique. It protects wood by means of controlled carbonisation on the surface. This carbonised layer extends its useful life to more than 50 years outdoors. With good maintenance, it can even reach nearly 100 years.
In addition to being durable, it is a sustainable method. It does not use chemicals and has a low environmental impact. Its aesthetic appeal is undeniable. It offers everything from deep, elegant blacks to softer, more nuanced textures. This achieves a balance between tradition and contemporary design.
When yakisugi is applied to thermo-treated wood at 212ºC, the benefits are multiplied. Thermo-treatment provides internal stability. The charred layer adds protection against the sun, rain and fungi.
The result is a material that combines the best of both processes. Highly durable, stable and low-maintenance cladding and decking. Re-oiling with pigmented oil is optional to preserve the colour. If left to age to a natural grey, durability is not affected.
USES AND COMMON APPLICATIONS
Thermo-d wood is a versatile and reliable material that performs well in demanding environments.
Most common applications:
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- Ventilated façades
- Outdoor terraces and decking, including those adjacent to swimming pools or hot tubs
- Fences and enclosures raised above the ground
- Garden and terrace furniture
- Interior cladding in spas and saunas
SUSTAINABILITY
The thermo-d process uses only heat and steam, without chemicals or biocides. This reduces harmful substances on site.
Furthermore, as it requires less maintenance and fewer treatments over time, its overall environmental impact is competitive with unmodified wood, even when considering the extra energy consumed in its manufacture (Kutnar & Burnard, 2014 – BioResources).
CONCLUSION
Although heat treatment can be applied to many species, at Zenwood we mainly work with Finnish pine/spruce because of its quality and stability.
This process transforms common woods into materials with properties comparable to hardwoods such as oak: greater durability, stability and resistance to outdoor conditions.
To ensure a good result, the treatment must always be carried out by qualified professionals. At Zenwood, we are specialists and can advise you on everything from choosing the species to the final finish.
Would you like to see for yourself? request your free kit of heat-treated wood and yakisugi samples and discover how they can transform your façade, terrace or garden.
