Thermowood is wood that is thermally modified in high temperature (212 degrees) and only using water vapor, no additional chemicals, in the process. This Thermowood process changes the properties of wood, giving it much better dimensional stability and higher durability against fungi and molds, which makes it an ideal material for exterior cladding. Good design and correct installation are essential to ensure a long service life of the wooden façade. Here are a couple of tips for the planning process to keep in mind.
1.Study the climate
The first step in the design process is to study the climate of the building site, because this directly influences the behavior of the façade. Exposure to the sun, humidity, rain, wind, snow and hail are key factors in the aging process of wood exposed to weathering. For example, the more solar radiation a façade receives, the sooner the signs of photodegradation will be noticed on it. That is why a south-facing façade will always turn grey faster than the north-facing façade. Another example is a critical place where a large amount and frequency of rainfall is combined, with high ambient humidity and mild temperatures, in which rot fungi live more comfortably. As there is a greater risk of development of xylophagous fungi, in these areas special attention should be paid to the design of the façade and its details by putting into practice the following practices: promote a rapid evacuation of rainwater, avoid the accumulation of water and facilitate drying through ventilation.
2. Cladding configuration
The second step is the selection of the cladding configuration. When choosing the configuration of the cladding, the aesthetic aspect is important, but criteria such as the climate of the area or exposure to the sun and rain should not be left aside. For example, in a horizontal cladding, due to gravity, water evacuates more slowly than in a vertical façade. There is an increased risk of water seeping through the gaps and leading to mold problems and deformations. On the other hand, the installation is usually simpler and cheaper in horizontal configuration. The vertical cladding is optimal because it follows the general direction of rain and is easier to clean.
3. Select the profile
After these first two steps it is time to choose the type of profile that fits into the project. There is a wide range of products on the market, from simple battens, brushed on four sides to more elaborate panels, imitating battens or for example imitating waves (Luna Aalto (in the photo)). This choice depends not only on aesthetics, but also on dimensions, prices, delivery times, etc. On facades with horizontal battens, it is recommended to use profiles with slope visual questions on the upper edge (e.g. Luna Parallelogram) to facilitate the evacuation of water and avoid the accumulation of dirt.
4. Decide the substructure
After selecting a profile, we should think about the needed substructure, the part where the Thermowood battens/boards will be fixed. The options are to use either wooden battens (pressure treated wood or durable wood) or sub-structure made of steel (stainless steel or aluminum).
It is recommended to protect the structure from water and air leaks by means of a breathable waterproof barrier, impermeable to rainwater and breathable to water vapor, with its joints taped according to the manufacturer’s instructions. This barrier is laid before the installation of the support battens.
In the case of thermally modified wood it is recommended to fix the boards in every 60 cm with a stainless steel screw (if the boards have a width of equal to or less than 140 mm) or with two screws if the boards have a width greater than 140 mm. The ventilation chamber should be at least 42 mm deep in the case of horizontally installed boards and 30 mm + 30 mm (= 60 mm) in the case of double battens (vertical boards). Battens should be made of pressure treated timber or of a durable species for use class 3.2. and have sufficient mechanical strength to withstand the weight of the cladding.
THREE RULES TO FOLLOW WITH LUNAWOOD CLADDING
As for the constructive details of the cladding, there are some rules that must be respected so that the cladding lasts long in good condition.
Rule nro. 1: Ensure ventilation and drainage between the structure and the cladding. The key to having a wooden façade that lasts a long time is to have enough big cap for ventilation and drainage between the structure and the cladding. In the case that we want to use the thermally modified wooden boards in vertical orientation, we will need double battens (first in vertical, and then horizontal) to guarantee proper ventilation and drainage.
Rule nro. 2: Protect the edges of the boards. It is advisable to protect the edges of the battens/boards when they are fully exposed. The best way is to protect them by means of an aluminum or wood profile that can be changed if needed. Always leave gaps for ventilation (see the drawing).
Rule nro. 3: Separate the cladding from the ground. In pavements that favor the splash of rainwater and in areas with high rainfall, the façade profiles must be separated from the ground by a minimum of 300 mm. By placing a perimeter mantle of gravel, the façade profiles can be placed closer to the ground, respecting a minimum distance of 200 mm. In case you want to reduce the separation with the ground, it is recommended to use a horizontal façade configuration that allows the replacement of the lower boards if necessary.
To prevent the entry of insects and small rodents into the ventilation chamber, it is recommended to install an anti-insect mesh on facades with a closed joint (facades with tongue-and-groove profiles or facades without space between battens).
The design of the façade and its details must be thought in detail to facilitate the evacuation of rainwater from the façade surface. It is advisable to avoid horizontal surfaces where rainwater can accumulate, especially in places where it rains a lot or is very cold and condensation water accumulates constantly.
For more information on how to design and install Lunawood Thermowood, please contact Lunawood Architect and Design Support
The author 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.