4.1 Experience has shown that uncontrolled water entry into thermal insulation is the most serious factor causing impaired performance. Several ways exist by which water enters into an insulation system, the primary ones being diffusion of water vapor, air leakage carrying water vapor, and leakage of surface water. Application specifications for insulation systems that operate below ambient dew-point temperatures necessarily include an adequate vapor retarder system. A vapor retarder system is separate and distinct from the insulation, or is provided by the insulation itself when it is has adequate vapor resistant properties and all joints are sealed against water vapor intrusion, in which case a separate vapor retarder system is not necessary. For selection of adequate retarder systems to control vapor diffusion, it is necessary to establish acceptable practices and standards.

4.2 Vapor Retarder Function—The primary function of a vapor retarder is to control movement of diffusing water vapor into or through a permeable insulation system. The vapor retarder system in some cases is designed to prevent entry of surface water. When properly functioning as a vapor retarder, it will also serve as a barrier to air leakage.

4.3 Vapor Retarder Performance—Design choice of retarders will be affected by thickness of retarder materials, substrate to which applied, the number of joints, available length and width of sheet materials, useful life of the system, and inspection procedures. Each of these factors will have an effect on the retarder system performance and each must be considered and evaluated by the designer.

4.3.1 Although this practice properly places major emphasis on selecting the best vapor retarders, it must be recognized that faulty installation is likely to impair vapor retarder performance. The effectiveness of installation or application techniques in obtaining design water vapor permeance (WVP) performance must be considered in the selection of retarder materials.

4.3.2 It is impractical to specify an “as installed” permeance value because, due to the nature of field application, attainment of system permeance equivalent to the vapor retarder materials themselves is assumed not possible. The best approach is to specify an appropriate vapor retarder and insure that proper installation and sealing procedures are followed.

1.1 This practice outlines factors to be considered, describes design principles and procedures for water vapor retarder selection, and defines water vapor transmission values appropriate for established criteria. It is intended for the guidance of design engineers in preparing vapor retarder application specifications for control of water vapor flow through thermal insulation. It covers commercial and residential building construction and industrial applications in the service temperature range from −40 to +150°F (−40 to +66°C). Emphasis is placed on the control of moisture penetration by choice of the most suitable components of the system.

1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.

1.3 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.

1.4 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.