【国外标准】 Standard Guide for Specifying Water Vapor Transmission Material Properties of Water-Resistive Barriers and Air Barriers

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.1.1 This document provides guidelines for specifying water vapor transmission (WVT) properties for above-grade water-resistive barriers and air barriers (WRB/AB), typically installed between building structural components and cladding that compose the exterior side of building envelopes in North America.1.2 This guide applies to all types of water-resistive barrier and air barrier products, including multifunctional products, regardless of the manufacturing process, type of material, or installation technique.1.3 This guide provides general provisions for specifying and reporting the water vapor transmission properties of WRB/AB determined by standardized test methods, in accordance with in-service conditions these products typically experience within building envelopes.1.4 It is beyond the scope of this guide to optimize the water vapor transmission characteristics of WRB/AB for specific conditions of use. The specific conditions of use should account for variations in indoor and outdoor climates, cladding type, moisture storage capacity of cladding materials, thermal insulating measures for wall and roof assemblies, air movement, and vapor diffusion control strategies.1.5 This guide does not address proper installation and integration of WRB/AB with other wall and roof components.1.6 The values stated in inch-pound units are to be regarded separately as standard. Within the text, the SI units shown in parentheses are provided for information only. The values stated in each system are not exact equivalents; therefore, each system shall be used independently. Combining values from two systems may result in non-conformance with the standard. However, derived results can be converted between systems using appropriate conversion factors (see Table 1).1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.8 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.