Many changes have been introduced to the construction of building enclosures in an attempt to reduce the extent and severity of water penetration problems. One of these innovations has been the more prevalent use of self–adhesive bituminous membranes as a protected flashing material at interface and penetration details. As with any innovation, there is a need to be wary of unintended impacts of this change. With the focus clearly on keeping water out of the wall once in service, there can be a lack of recognition of the reduction of drying capability that is inherent in using vapor impermeable membrane within a wood frame wall that is intended to otherwise dry to the exterior. In addition, the focus on exterior moisture (rain penetration) fails to acknowledge that a variety of moisture sources can exist and potentially contribute to an unanticipated moisture problem.
This bulletin applies to wood-frame wall assemblies designed with interior vapor control membranes, and relatively permeable water resistive membranes, as applicable to mixed and cold climates.