A limit state design (LSD) approach for comparing relative drying performance of wood-frame envelope systems with full-scale lab testing

The capacity of drying out moisture accumulated within stud cavities, especially moisture due to rain penetration, is an important characteristic affecting the performance and durability of building envelope systems. This paper introduces a new approach for evaluating such drying performance based on the concepts and procedures of Limit State Design (LSD) used in structural engineering. For a well performing envelope, the moisture load must be less than the drying capacity of the system. The drying capacities are obtained through full-scale experiments that utilize moisture loading derived from a moisture source (water tray) placed at the bottom of the stud cavity. In moving out of the cavity, part of the moisture will be absorbed by the materials surrounding the stud cavity. When any part of the wall specimen reaches 20% MC, the cumulative evaporation from the water tray is termed as the ICEA (in-cavity evaporation allowance) of that wall system. The ICEA value is dependent on the envelope configuration and is a good indicator of the drying performance of that wall system. By comparing ICEA values of wall systems with their respective moisture loads, those envelopes having ICEA values higher than the amount of rain penetration calculated from prevailing driving rain and faults in the envelope are deemed to have adequate drying capacity.