The vapor diffusion resistance and air permeance of masonry and roofing systems

Several building parts contain layers composed of separate elements that are mortared together or simply interlock or overlap. Typical examples are masonry walls, brick veneers, tiled roofing systems and slated roofing systems. The mortar joints, the interlocks and overlaps should affect the water vapor diffusion resistance and the air permeance of the composite layer, at least in comparison with the vapor and air flow properties of the pure elements. However, little information is found in literature that allows a quantification of the differences. The article describes a purpose designed test method for measuring the water vapor diffusion resistance of composite layers. It comments on the results and also gives data for the air permeances, measured with a pressure box. From the results and data, it is clear that mortared joints, interlocks and overlaps decrease the water vapor diffusion resistance and increase the air permeance of a composite layer compared to the pure material. In fact, mortared joints, interlocks and overlaps act as preferential paths for water vapor and air mitigation. As a consequence, using composite layers as outside finish in thermally insulated building parts, as done in pitched roofs and cavity walls, diminishes the interstitial condensation risk, accelerates drying of the cladding when wetted by wind-driven rain and affects the effectiveness of outside air cross ventilation below or behind the finish.