Parametric Study of Moisture and Heat Transfer in a New Rain-Screen Stucco Wall

Abstract

The cost of repairs to leaky condominiums in southwestern British Columbia (BC) has been estimated at one billion dollars. Stakeholders have been actively trying to address the problem. Providing a drainage cavity (rain-screen) in wall claddings is emerging as the promising solution to offer a second line of defense against moisture ingress and is currently advocated by municipal authorities, designers and manufacturers of building envelope systems. However, it is not yet clear which assembly of materials will provide an optimal solution or what size of drainage cavity should be used. Designers do not agree on whether the cavity should be pressure equalized, pressure moderated or vented and to what extent. The effects that the cavity will have on the drying and energy efficiency of walls also remain open questions. Since stucco has been a major cladding material in BC, extensive research to design an effective drainage cavity stucco cladding system was carried out. A detailed computer study using the “MOIST 3.0” computer model was conducted to analyze the moisture and heat transfer in a new drainage cavity system versus a conventional stucco wall. The ASHRAE WYEC hourly weather data for the city of Vancouver along with the variable indoor relative humidity model were used as input. Results show superior characteristics of the proposed drainage cavity cladding system. Several residential projects, including multifamily buildings, have been constructed using the new rain-screen system, which is exhibiting good performance.