The natural growth process of wood relies on rapid transportation of water. It also retains these hygroscopic properties later. Wood is therefore particularly sensitive to humidity. Timber swells when it is too moist, and warps when dried. These dimensional changes do not develop equally in every direction, as wood swells or shrinks tangentially more than radially (in a ratio of approx. 2:1).

The uneven dimensional changes and the extreme effect on the sap wood compared to core timber caused by alterations in water content lead to deformation and, frequently, crack formations. Timber deformation naturally depends on the type of wood, unit weight, the annual trunk ring development and fibre direction. Practical selection of the cutting technique and cutting configuration can therefore remedy shrinkage and swelling.

The moisture content of wood is equivalent to that of ambient air where it is not subject to direct humidity influences (e.g. rain). The ideal moisture content of wood during processing is 9 – 12%. These values are automatically attained at a relative air humidity of 50 – 60%. An equilibrium in terms of humidity therefore exists between air and timber. This relative air humidity is only characterised by minimal falls or rises during the summer months, even in enclosed spaces. The situation is different during hot periods. Relative air humidity falls regularly to 30 – 20% where the air is heated.

In situations where such a low value is created, particularly as a result of a rapid fall in air humidity, there is an acute risk of damage to wood during storage and processing.

Recommended air humidity values in the timber industry