Controlling air

by sadia_badhon | December 13, 2019 11:08 am

FAILURES
Deborah Slaton, David S. Patterson, AIA, and Jeffrey N. Sutterlin, PE

In most climate zones within the United States, a dedicated air barrier is required by code as part of the building enclosure to control and manage airflow between the conditioned interior and exterior environments. To be effective, the air barrier needs to maintain continuity, including interfaces with other components of the enclosure and penetrations. Unlike a vapor retarder, even small voids can significantly impact an air barrier’s performance.

[1]

An air barrier often consists of several materials installed as an assembly. According to the American National Standards Institute/American Society of Heating, Refrigerating and Air-Conditioning Engineers/Illuminating Engineering Society (ANSI/ASHRAE/IES) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, as referenced in the International Energy Conservation Code (IECC), the permeance of air barrier materials must not exceed 0.02 L/s/m² (0.004 cfm/sf) at a pressure difference of 75Pa (1.57 psf), or 0.2 L/s/m² (0.04 cfm/sf) for the assembly (which includes accessories such as sealants, tapes, etc.)—maximum air permeance for the entire enclosure (or air barrier system) is stipulated at 2 L/s/m² (0.4 cfm/sf) under the same pressure differential. This standard further requires that all components of the air barrier be clearly identified and that interconnections and penetrations be detailed on the construction documents. The standard also offers a list of conditions requiring specific attention during installation. However, even with this level of detail, challenges are often encountered in achieving an effective air barrier due to seemingly innocuous components such as utility and other penetrations.

For example, at a recently built natatorium in the northeast, the dedicated fluid-applied air barrier located on the exterior of the sheathing was integrated with the perimeter of the junction boxes for exterior light fixtures and electrical outlet penetrations (a one-piece boot was also used to seal the enclosures). Although this detail created an effective integration of the air barrier with the junction box, the installation did not account for the unsealed electrical conduit serving the box. Interior moisture entering the junction box via the open conduit had resulted in condensation and moisture-related damage to electrical devices during the first winter following occupancy.

[2]

Similarly, at a building in the southeast, corrosion and water accumulation were observed within an electrical panel. It was determined one of the electrical conduits feeding the panel served equipment within the unconditioned crawlspace of the building. As the structure was negatively pressured relative to the exterior environment, this conduit provided a direct path for unconditioned crawlspace air to enter the panel, causing condensation within the panel during humid summer months when the building was in a cooling mode. Even though steps were taken to seal the conduit penetration at the floor slab above the unconditioned crawlspace, the inside of the conduit was unsealed, making it a virtual ‘straw,’ drawing moist air into the negatively pressured building.

As illustrated in these two examples, it is important to recognize all air paths need to be addressed when specifying, detailing, and installing an effective air barrier.

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

Deborah Slaton is an architectural conservator and principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation and materials conservation. She can be reached at dslaton@wje.com[3].

David S. Patterson, AIA, is an architect and senior principal with WJE’s office in Princeton, New Jersey. He specializes in investigation and repair of the building envelope. He can be reached at dpatterson@wje.com[4].

Jeffrey N. Sutterlin, PE, is an architectural engineer and senior associate with WJE’s Princeton office, specializing in investigation and repair of the building envelope. He can be reached at jsutterlin@wje.com[5].

Source URL: https://www.constructionspecifier.com/controlling-air/

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