Air Vapor Barriers: When in Doubt, Box it Out

slaton patterson sutterlinFAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE

Air vapor barriers (AVBs) are critical in controlling air and moisture transport within building enclosure assemblies. Ignoring bulk water penetration, moisture-related issues experienced within the building enclosure are more commonly the result of moisture transport through air movement via discontinuities in the AVB than vapor diffusion through the wall assembly.

Even small voids in an air barrier can allow transport of significant amounts of moisture when air pressure differences exist across the barrier’s plane. For example, in a pressurized building under normal heating or cooling conditions, the moisture transported through a 25.4 mm (1 in.) square hole can be up to 100 times greater than the amount of vapor diffusion through a 1220 x 2440-mm (48 x 96-in.) sheet of gypsum board sheathing.1 Therefore, the following example focuses on the air barriers aspect of AVBs where failure to ensure air barrier continuity can significantly undermine AVB’s effectiveness.

Successful installation of the air/vapor barrier (AVB) was extremely difficult at the complex connection created where multiple structural members met at a single point. The issue was resolved by ‘boxing out’ the connection to provide simpler transitions and terminations of the AVB.Photos courtesy Jeffrey N. Sutterlin

Installation of the AVB was difficult at the complex connection created where multiple structural members met at a single point. The issue was resolved by ‘boxing out’ the connection to provide simpler transitions and terminations.

The AVB’s critical role in preventing moisture-related problems requires attention to difficult interface conditions that may be encountered in the real world, such as complex structural details or multiple conduit, pipe, wiring, or duct penetrations. Often overlooked during design, these conditions are commonly addressed during construction through copious quantities of sealant or mastic that are of poor geometry and replete with voids. Improperly executed, such details can be vulnerable to air leakage (moisture transport), and are not conducive to long-term serviceability.

An institutional building located in the Northeast (i.e. Climate Zone 5A, Cool−Humid) was positively pressured, with interior conditioned space maintained at elevated relative humidity (RH) levels of 50 to 60 percent. As designed, a vapor-impermeable air barrier was installed along the demising partition separating interior conditioned space from unconditioned attic space to avoid passage of moist interior air into the attic space, which could result in condensation during colder winter conditions. A complex structural connection joining several diagonal steel members bisected the partition at the wall base, creating a challenging interface with the AVB.

Due to the complexity of the connection and limited access around each member in the example illustrated, integrating an effective air barrier was extremely difficult to execute. To simplify the AVB installation at this condition, the entire structural connection was ‘boxed out’ with gypsum sheathing extending beyond the complex geometry and congestion at the connection, allowing for a more constructible (and reliable) integration of the AVB membranes.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect those of the CSI or The Construction Specifier.

1 See Joseph Lstiburek’s Research Report 0412, Insulation, Sheathings, and Vapor Retarders (Building Science Press, 2006). Visit www.buildingscience.com.

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com.
David S. Patterson, AIA, is an architect and senior principal with WJE’s Princeton, New Jersey, office, specializing in investigation and repair of the building envelope. He can be e-mailed at dpatterson@wje.com.
Jeffrey N. Sutterlin is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be contacted via e-mail at jsutterlin@wje.com.

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