It can be often heard that wooden windows "breathe", whereas PVC and aluminium windows don ´t. There is only a shade of truth in this statement as wooden windows breathe at most because of poor sealing and not due to the breathing of wood. The manufacturers of wooden windows pay great attention to ensure the best possible airtightness, water tightness and wind resistance. In addition, we strive to produce wooden windows with high-quality surface protection, which ensures long lifetime without the repainting of windows. This can be achieved by thick and elastic surface coatings that close the wood surface. How and why would wooden window breathe therefore?
As wood is a porous material, the air permeability through the wood is possible, but it is highly dependent on the anatomical structure of the wood, its density and moisture content, and the direction of the air transport. As an example, we can take balsa wood which is considered to be the commercial wood species with the lowest density (cca. 150 kg/m3) and Norway spruce (cca. 450 kg/m3), which is most commonly used wood species in Slovenia for the production of wooden windows. If we blow (with mouth) into the small and short piece of balsa wood in the direction of wood cells (axial direction), we could feel slight air on the other side of sample as air will actually pass. That can´t be done with Norway spruce, especially in the case of blowing perpendicular to the cells (lateral direction), as the air should pass through a number of cell walls.
The latter is a practical example for wooden windows, as the air should travel in the direction perpendicular to the wood cells, either from the interior to the outside, or vice versa. The situation is further aggravated because wooden window profiles are coated with synthetic coatings and made of laminated wood bonded with adhesive. This means that, in addition to air transport through the wood itself, the air must pass through two layers of cured coating and additionally through at least two cured adhesive joints that connect the wooden lamellas of the window profile.
Do wooden windows really breathe? Consider yourself.