The Rehab Guide, Volume 2: Exterior Walls - ToolBase Services

5VAPOR RET ARDERS& AIR INFILTRATIONBARRIERS5.1VAPOR RETARDERESSENTIAL KNOWLEDGE32Vapor retarders first appeared in building construction in the 1920s. Early theories held that moisturevapor will migrate from a region of high concentration towards a region of low concentration along a linearpath. The amount of moisture transfer is dependent on the differences in concentration and the vaporpermeability of the membrane separating the two regions. This is the theory of vapor diffusion, whichviewed the flow of moisture vapor directly analogous to the conductive flow of thermal energy. In this theory,air movement, and the moisture propelled by it, were not considered to be major factors. In the early1950s, Canadian research found that air movement was the primary mechanism of moisture vapor migration.Without active air infiltration control, vapor retarder barriers become ineffective.Current theory on vapor retarders indicates that both air infiltration and direct diffusion playsignificant roles in the transfer of moisture vapor and, therefore, both must be accounted for. Effectivevapor retarders must have a water vapor permeability not exceeding 1.0 grains per hour per square footper inch of mercury vapor pressure difference (referred to as 1.0 perms), and must be installed in sucha manner as to prevent air leaks at joints and laps.Although the issue of what makes a vapor retarder effective is generally settled, controversy stillremains as to where to install it, if at all. From this standpoint, the authority on the subject is the 1997ASHRAE Handbook of Fundamentals, which has more to say on the topic than any of the model codes. Inwhat is defined as heating climates (4,000 heating degree days, base 65ºF, or more), vapor retarders belongon the interior side of the insulation. In warm, humid, cooling climates (Florida and Gulf Coast) where moisturevapor transfer conditions are effectively reversed, vapor retarders are best placed close to the exterior.In mixed climates (not fitting either of the above definitions), the vapor retarder should beplaced to protect against the more serious condensation condition, summer or winter. If in a mixed climatethe winter indoor relative humidity is kept below 35 percent, a vapor retarder at the interior side ofthe insulation is usually not required, and an exterior vapor retarder strategy is most effective. Where winterinterior humidity is not controlled or if a humidifier is used, an interior vapor retarder is most useful.Vapor retarders should never be placed on both sides of a wall. Where a vapor retarder isemployed, the opposite wall surface must provide a permeable surface to allow drying to occur. Thus, inhot, humid, cooling climates, where a vapor retarder is employed at the exterior, the interior wall surfacesshould be permeable. No vapor retarder paints, kraft-faced insulation, or vinyl wall coverings should beused. Conversely, in northern heating climates, with interior vapor retarders, the exterior wall coveringsshould be vapor permeable.The primary purpose for installing a vapor retarder in residential rehabilitation is to minimize