Standard Guide for Specifying Water Vapor Transmission Material Properties of Water-Resistive Barriers and Air Barriers

Abstract

4.1 As the building industry shifts towards performance-based design, specification of material properties consistent with anticipated in-service conditions becomes paramount to the design process. When specifying water vapor transmission properties, it is important to identify water vapor transmission properties for WRB/AB products that are measured under test conditions relevant to anticipated in-service conditions. This guide provides a performance-based framework for characterizing the water vapor transmission properties of WRB/AB. 4.2 When specifying WRB/AB, water vapor permeance is an important attribute to consider for proper moisture management and functioning of wall and roof assemblies in service. In North America, water vapor transmission properties of water-resistive and air barrier materials are traditionally tested in accordance with Test Methods E96/E96M. This guide adopts the ASTM E96/E96M test methods as a primary source of information for water vapor transmission properties of WRB/AB unless otherwise instructed by the design professional. 4.3 Most standard test methods rely on a limited set of steady-state testing conditions for evaluating the water vapor transmission properties of materials. Test conditions used to measure and report water vapor transmission values of WRB/AB should represent the in-service conditions of the tested material as closely as possible (that is, should cover the range of temperature and relative humidity conditions the products will experience when installed in wall and roof assemblies). The water vapor permeance of many WRB/AB materials can vary by more than an order of magnitude when tested for ranges of temperatures and relative humidity expected in service. For this reason, WVT properties over the full range of environmental conditions that the material will most likely experience in service should be used or evaluated when specifying a material or assembly design for a specific project.