Thermowood Cladding

Heat treated timber (Thermowood)
Surface treatment is not essential.
30 year UK service life
Please visit http://www.thermowood.fi  for information about Thermowood

Thermowood introduction

It has been known for centuries that burning the surface of wood in open fire will make it more durable in exterior use. Even the Vikings used this method in
outdoor structures such as fences.
Heat treatment of wood was scientifically studied by Stamm and Hansen in the
1930s in Germany and by White in the 1940s in the United States. In the1950s,
Germans Bavendam, Runkel, and Buro continued research into the subject.
Kollman and Schneider published their findings in the 1960s, and Rusche and
Burmester in the 1970’s. More recently, research work was carried out in
Finland, France, and the Netherlands in the 1990s. The most intensive and
comprehensive research work was conducted by VTT in Finland. Significant
practical research is also being done by YTI (the Institute of Environmental
Technology).
Thermowood is manufactured using a method developed by VTT. The wood
material is heated to a temperature of at least 180 degrees Celsius while it is
protected with steam. Besides providing protection, the steam also affects the
chemical changes taking place in the wood. As a result of the treatment,
environmentally friendly Thermowood is created. Its colour darkens, it is more
stable than normal wood in conditions of changing humidity, and its thermal
insulation properties are improved. If carried out at a sufficiently high
temperature, treatment also makes the wood resistant to decay. On the other
hand, this has a decrease in the bending strength.

Changes to wood structure

Heating wood permanently changes several of its chemical and physical
properties. The change in properties is mainly caused by thermic degrading of
hemicelluloses. Desired changes start to appear already at about 150 ºC, and
the changes continue as the temperature is increased in stages. As a result,
swelling and shrinkage due to moisture is decreased, biological durability is
improved, colour darkens, several extractives flow from the wood, the wood
becomes lighter, equilibrium moisture content decreases, pH decreases, and
thermal insulation properties are improved. However, thermowood’s rigidity and
strength properties are also changed.