The Performance of Rainscreen Walls in Coastal British Columbia

Abstract

This thesis examines the widespread moisture problems which emerged over the past twenty years in buildings throughout coastal British Columbia, commonly known as ‘leaky condos’. A literature review of building physics and a historical review of wood-frame construction in North America provide background for this review.

The purpose of this work is to report and interpret the performance of rainscreen walls in the coastal climate of Vancouver BC, based on extensive field data from five local buildings constructed or rehabilitated with rainscreen wall assemblies. Hygrothermal data was collected within exterior walls, and corresponding environmental data was recorded for each building. Driving rain loads at the five buildings across the city are calculated and compared to Vancouver airport data. Site factors are shown to have a significant impact on driving rain load, wind speed and direction.

The WUFI 4.1 hygrothermal model was compared with the field data collected and found to be accurate at predicting past performance. Applying this validated model to each wall assembly, further simulations were performed to determine the impact of boundary conditions and assembly details on wall performance.

Field measurements and modeling show that ventilated and drained claddings (i.e. rainscreen) reduce the sensitivity of wood frame buildings to moisture damage. Ventilation of the cladding is shown to be particularly important and natural buoyancy forces (from temperature and humidity differences between cavity and exterior) are usually sufficient to provide good drying. Exterior insulation is shown to further improve rainscreen wall performance by increasing the drying potential of the sheathing to both the exterior and interior.

Additional work performed included material testing of fiberglass-faced gypsum sheathing and air-leakage testing of individual suites in the monitored buildings. Elevated interior humidity, resulting from inadequate ventilation, is shown to be exacerbated by inter-zonal air-flow in multi-unit residential buildings.