Thermally modified wood in the Mediterranean climate – Abiotic factors

Lately wood is being the protagonist in many projects around the world. In Mediterranean climate wood is used as a structural element but it is also used in decorative applications and increasingly also in exterior. There are still many doubts and, also quite a bit of ignorance regarding the durability, maintenance, and treatments for wood outdoors in the Mediterranean climate.

In this a article we want to clarify some doubts about this topic. We concentrate on abiotic (‘=physical rather than biological’); factors that cause aging and natural deterioration of the thermally modified wood.

Abiotic factors

The concept “Weathering”, the aging of wood, is a complex process, which is caused through the contact of materials with the earth’s atmosphere, the fauna and flora of a place, the water, and the sun. We are talking about a mainly superficial effect, and therefore it does not affect the mechanical resistance of the wood, as long as the design, installation and surface treatment are correctly executed.

The impact of weather on Thermowood

One of the causes of aging is the water from the rain that wets the surface of the wood. The water vapour is collected directly by absorption by the walls of the cells. The difference in moisture content between the interior and the surface layer of the wood causes internal tensions in the piece, and if it is not balanced, this causes dimensional changes in the wood.

This effect is very reduced in Thermowood thanks to the thermal modification, after which the timber’s hemicellulose content is significantly lower, reducing the concentration of water-absorbing hydroxyl groups and ultimately improving the dimensional stability of Thermowood. However, there are ways to minimize the effect of water, for example using wooden profiles that allow water to escape easily, installing the battens vertically, leaving enough distance (min. 300 mm) between the ground and the wood, and end-sealing of the cladding boards.

Photo: Marià Castelló's design in this Ca l’Amo project in Ibiza solves several issues: It protects the end grain of the wood and leaves a lot of distance between the wood and the ground to prevent water from splashing and wetting the wood.

Another important factor is solar radiation. Wood exposed to sunlight undergoes colour changes that go through different phases depending on the climate, the wood species, and the orientation. When wood is dried by the sun, the wood extracts, including tannins, come to the surface and, when in contact with oxygen, oxidize to form brown spots. In the case of Thermowood, tannins, like most extractives, evaporate during thermal modification and thus the formation of these spots in Thermowood is minimized.

Thermowood has initially slightly dark brown, “toasted” color, which gradually turns to grey, if exposed to sun. The graying is less intense on the facade facing to the North, as it receives less solar radiation compared to the other facades. The design, orientation of the boards or battens, and especially the geography (climate) influence at the rate of graying. The more clean-cut the design, the more homogeneous the graying.

Nursery Savannen in Denmark by Sofie Peschardt/ Nordic – Office of Architecture. Photos: Imarken, Kirstine Mengel. Years 2019 and 2020.

The solar radiation, along with the wind, does not only change the color of the wood, but also degrades the lignin, which is the “glue” that connects the wood fibres. The effects of this degradation, combined with other natural factors, such as mechanical abrasion of the particles dragged by the wind, can be detected earlier in lignin than in cellulose. Because of this, exposed wood gets this “rough” surface, where the spring wood wears off quicker than the autumn wood.

All these factors mean that the colour changes are not homogeneous, and the process can take quite a long time. If this aging process is not accepted by the architect or end customer, surface treatments for wood can be used. Surface treatment will protect the wood from previously mentioned factors and maintain the original colour for longer time. There are many types of products available on the market, but this is already a topic for another article.

Photo: Spring wood has been wearing off quicker than the denser autumn wood, giving the Thermowood a rough, yet rustic surface.

Effect of salt on Thermowood

In the coastal areas we also cannot forget the effect of salt on wooden surfaces. Many building materials are known to suffer from contact with salt water, and for example metal and concrete structures deteriorate rapidly in coastal areas. Wood has the advantage that it does not corrode, does not produce alkaline alterations with aggregates, does not generate problems of internal pressure by crystallization of salts, nor is it deteriorated by the chemical aggressiveness of sulfates. The biggest problem for wood in coastal areas is the high humidity of the environment, and without proper treatments the wood can suffer from fungal attacks due to changes in humidity in the wood. Thermowood is also ideal for these areas, as it does not take much moisture in and has a good resistance against fungi.

Contact us for further information

The writer is Dipl.Ing. (MSc)) Julia Ahvenainen, who has long experience in the research of the wood structure and wooden industry. She works as Project Business Manager at Lunawood and serves architects and designers in Spain.

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