Volume 5, Issue 7 (July 2008)
Evaluating Drainage Characteristics of Water Resistive Barriers as Part of an Overall Durable Wall Approach for the Building Enclosure
The most recent model residential building code has been modified to require increased use of water/weather resistive barrier (WRB) materials in construction, and to require some means of draining water from the building enclosure. However, “drainage” performance is not defined, and the Code is unclear about which currently available WRB products and design approaches help provide a durable water-resistant exterior wall enclosure. The present work is a qualitative, “order of magnitude” study of the drainage characteristics of various types of WRB materials (felts, housewraps, drainage wraps, drainage boards, and furring strips), and is a “first step” toward developing an overall durable wall approach for the building enclosure. The ASTM E2273 drainage efficiency test was used in 40 wall assembly mockups to evaluate 11 WRB materials in 8 design configurations. Traditional WRBs and housewraps provided little or no drainage capability to the exterior wall designs tested. Drainage-enhanced housewraps provided an improved level of drainage, but they still retained water. The retained water can migrate through fastener holes to the underlying construction. Best drainage performance was obtained by using WRBs with furring, drainage mats, and profiled sheets (drainage boards). These overall results are in general agreement with similar research by others. These results can guide designers and builders in the proper selection and use of such materials, which should be used as part of an overall durable wall approach to protecting the building enclosure.