Pressure Equalization in Rainscreen Wall Systems

Rain can enter only if a combination of the following three conditions are present: rain water deposited on the wall, holes and cracks offering water leakage paths inwards and a force or a combination of forces to drive water inwards. Strategies for rain penetration control must entail the control of one or more of these contributing factors. Various approaches to rain penetration control are currently used in the building industry. These range from single-line-ofdefence assemblies (commonly called faceseal walls) to multiple-defence assemblies such as those that incorporate a rainscreen, a drained air space and a water-resistant membrane.1 The pressure-equalized rainscreen (PER) wall design is one of these multi-defence approaches. It is based on the open rainscreen principle,2 which aims to control all forces that can drive water into the wall assembly, i.e., air pressure difference, gravity, surface tension, capillary action, and rain drop momentum. Of these forces, air pressure difference is often a dominant one with the potential to drive a considerable amount of rainwater into the wall assembly. This Update therefore focuses on the control of air pressure difference across the rainscreen, and the particular elements of wall assemblies instrumental in obtaining such control.