Air Leakage, Moisture Migration, and Condensation in a Building Enclosure

Abstract

Water vapor within a building can lead to condensation on or within building envelopes via airflow/air leakage, vapor diffusion, and on exposed surfaces. Water vapor inside a building naturally equalizes between adjacent environments via diffusion, and because air naturally flows towards regions of lower air pressure. Each transport mechanism requires different design approaches, since vapor diffusion calculations are relatively straightforward but predicting air leakage is much more difficult and often depends upon construction and workmanship issues rather than simple material properties. Condensation on interior surfaces is a more visible issue than the often-concealed issues of vapor diffusion and air leakage, but can still be difficult to diagnose and resolve. This session presents multiple case studies and practical examples to demonstrate the nature of these issues and illustrates how design and construction issues can cause both short- and long-term problems. Specific issues such as exterior icing, condensation on fenestration, concealed condensation within interior and exterior walls, and deterioration/failure of roofing systems are discussed.