Untersuchungen zur Wasserdampfdurchlässigkeit von Beschichtungen auf Holz für den Außenbereich

Investigations on water vapour permeability of wood coatings for exterior use.

Four commercially available coating systems including primer and top coat for façades and windows were applied to spruce samples. The systems were based on alkyd resins, acrylates, hybrid systems, modified PU systems, co-polymers and mineral binders. Each of the four systems was applied with 3 different thicknesses for measuring the water vapour resistance. The measurements were done in the dry-cup and wet-cup mode and with different orientations of the sample in the diffusion flow. The µ-values and the diffusion coefficients show a clear correlation to the thickness of the coating. Further an increase of the moisture content reduces the vapour resistance. Systems based on alkyd resin are very closed and block the water vapour especially at low moisture contents (4-6 %). On the contrary systems based on mineral binders are much more open and let the vapour pass through. This dependency on the binder system is not obvious for higher moisture contents (22-24 %). In this case all analysed variations were vapour permeable.