4.1 Experience has shown that uncontrolled water entry into thermal insulation is the most serious factor causing impaired performance. Water entry into an insulation system may be through 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 should include an adequate vapor retarder system. This may be separate and distinct from the insulation system or may be an integral part of it. For selection of adequate retarder systems to control vapor diffusion, it is necessary to establish acceptable practices and standards.

4.2 Vapor Retarder Function-Water entry into an insulation system may be through diffusion of water vapor, air leakage carrying water vapor, and leakage of surface water. 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 alone is seldom intended to prevent either entry of surface water or air leakage, but it may be considered as a second line of defense.

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 techniques can impair vapor retarder performance. The effectiveness of installation or application techniques in obtaining design water vapor transmission (WVT) performance must be considered in the selection of retarder materials.

4.3.2 As an example of the evaluation required, it may be impractical to specify a lower "as installed" value, because difficulties of field application often will preclude "as installed" attainment of the inherent WVT values of the vapor retarder materials used. The designer could approach this requirement by selecting a membrane retarder material that has a lower permeance manufactured in 5-ft (1.5-m) width or a sheet material 20 ft (6.1 m) wide having a higher permeance. These alternatives may be approximately equivalent on an installed basis since the wider material has fewer seams and joints.

4.3.3 For another example, when selecting mastic or coating retarder materials, the choice of a product having a permeance value somewhat higher than the lowest obtainable might be justified on the basis of its easier application techniques, thus ensuring "as installed" system attainment of the specified permeance. The permeance of the substrate and its effects on the application of the retarder material must also be considered in this case.