Wooden window frames absorb moisture from rain, wet snow, and condensation. If the water content in the wood goes above 20%, it becomes subject to biological attack. Rapid drying, under the influence of sun, causes cracking through which mould spores can enter the wood. Particularly vulnerable to water penetration is the end-grain. The article describes tests on water movement in bare and painted wood, and permeability of free paint films and painted wood. Tables, graphs, references.