Wood is a hygroscopic material, it has the ability to adsorb or desorb water in response to the ambient relative humidity. Thus, the ambient air will affect the moisture content of the wood, and in turn, the dimension of the wood. If the wood itself is part of the air barrier in a construction, the shrinking and expansion can create gaps in the construction, for example in the window sill. In case of an air barrier consisting of a foil, the joints in the foil can be clamped by wooden joists, or the foil can be taped to wooden part. In both cases, the movement (shrinking, expanding) can cause air leakages, in particular since wood is an anisotropic material and will behave differently in different directions.
This project investigates the air leakages that occur in wooden buildings from seasonal variations in climate and from finished construction (first airtightness measurement) to equilibrium (when inbuilt moisture has dried out). Measurements are made in real buildings (full scale) and in laboratory (climate chamber). The full-scale measurements on buildings with wooden construction have shown magnitudes of up to 10% increase in air leakage (at 50 Pa) for the seasonal variations and the increase for reaching equilibrium in a newly built construction is 20%.