Thermowood Disadvantages and Real Lifespan: Know Before You Buy

Thermowood is usually sold on its strengths: stable in humidity, resistant to rot and warm in colour. Yet in German and English searches, some of the most frequent questions are about thermowood disadvantages and how long thermowood lasts, questions that the market rarely answers honestly.

Thermowood is natural timber treated at high temperature without oxygen. The process changes the chemistry of the wood; some of those changes are welcome, others come at a cost.

Four real disadvantages of thermowood

• More brittle: bending and impact strength drop after heat treatment. Thermowood is not suitable for load-bearing beams or structural framing, and its edges break more easily when cutting and screwing.
• Greying in the sun: the brown colour of thermowood is not UV-stable. Without oil or a protective finish, the surface turns silver grey under strong sun. This is not structural damage, but it changes the look.
• Fine surface checks: in direct sun and with large day-night temperature swings, hairline cracks appear on the surface and board ends.
• No defence against termites: heat treatment makes wood resistant to fungi, but not to termites. In affected areas, separate prevention is needed.

If termites have been reported in your area, read our article on termites and wood pests before installation.

Thermowood is not magic timber; it has learned to live with water, not with sun and load.

What decides how long thermowood lasts?

The International ThermoWood Association (Finland) defines the Thermo-D class for exterior use above ground: facades, cladding and decking fixed on a substructure with no direct contact with soil or standing water. That definition is the key to a long life. Thermowood buried in soil, or with water trapped beneath it, fails much sooner than expected.

Note that this classification was written for the cold, wet climate of northern Europe. In Isfahan and most of Iran, the main enemy is not moisture but intense sun and large day-night temperature differences. Here, a UV-protective finish and ventilation behind the boards play a bigger role in thermowood lifespan than they do in Europe.

How to control thermowood drawbacks

• Always pre-drill before screwing and use stainless steel fasteners; ordinary steel screws leave black stains on thermowood.
• If you want to keep the brown colour, apply an oil or finish with UV protection from day one and renew it regularly; we cover the method and intervals in oiling thermowood.
• Leave ventilation gaps between and behind the boards so water cannot sit; the principles are in ventilated wood facades.
• Do not use thermowood as the load-bearing structure; build the substructure from structural timber or metal.

Bottom line: who is thermowood right for?

If you want a facade, terrace deck or balcony cladding and accept that the wood will either turn silver over time or need oiling every so often, thermowood is a good choice. If you expect maintenance-free, load-bearing or termite-proof timber, thermowood alone will not deliver and the design needs to change.

Sources

• International ThermoWood Association — ThermoWood Handbook