The need for effective air barriers as part of the building enclosure assembly separating conditioned interior space from the exterior environment is becoming better understood within the industry from both a design and construction perspective; however, applying that same philosophy and diligence to semi-exterior enclosures is often overlooked. This is particularly true if the semi-exterior enclosure is significantly inboard of the primary exterior enclosure for the building.
According to ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Sites and Buildings Except Low-rise Residential Buildings, air barrier materials must have an air permeance not exceeding 0.02 l/s/m2 (0.004 cfm/sf) at a pressure difference of 1.57 psf (75 Pa), 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). ASHRAE 90.1 also defines semi-exterior building envelope as building elements that separate conditioned spaces from unconditioned space (e.g. a vented crawlspace or soffit) or that enclose semi-heated spaces through which thermal energy may be transferred to or from the exterior, unconditioned spaces, or conditioned spaces (e.g. a semi-heated storage space or interior loading dock). ASHRAE further requires all exterior and semi-exterior portions of the building envelope to be designed and constructed with a continuous air barrier that complies with the established design, installation, and testing requirements outlined for exterior wall assemblies.
The lack of an effective air barrier increases the potential for building performance issues, including uncontrolled moisture transport through the assembly (as air contains water vapor); reduced energy performance, human comfort, and acoustic attributes; increased condensation potential; and the passage of undesirable odors, gases, and fumes into the building interior.
One example of this problem is a laboratory in the southeastern United States that was experiencing increased levels of relative humidity on the interior during summer months, affecting human comfort and resulting in the formation of condensation on supply diffusers and within fume hoods and metal casework. An investigation determined the building was operating under a negative air pressure relative to the atmosphere and the air barrier detailing at several semi-exterior enclosure locations was incomplete. Inadequate detailing was most notable at the semi-exterior enclosure separating the conditioned laboratory from the vented soffit, allowing moisture-laden exterior air to be drawn into the negatively pressured interior space.
Another example is an institutional building in the northeastern United States where a large semi-conditioned loading dock was designed to be inside the primary building enclosure, and occupants complained about periodic unpleasant diesel exhaust odors. Similar to the example above, it was determined the conditioned interior of this building was operating under a slight negative air pressure relative to the loading dock space. In addition, penetrations in demising walls and the floor slab above the loading dock (all semi-exterior enclosure planes) were not effectively sealed, allowing exhaust gases to be drawn into the conditioned interior of the building.
These examples highlight the necessity for air barrier continuity in buildings beyond the perhaps obvious primary exterior enclosure.
Jeffrey Sutterlin, PE, is an architectural engineer and associate principal with Wiss, Janney, Elstner Associates (WJE) in Princeton, New Jersey. He specializes in the investigation and repair of the building enclosure, as well as peer review and consulting for new enclosure design. He can be reached at jsutterlin@wje.com.
David S. Patterson, AIA, is an architect and senior principal with Wiss, Janney, Elstner Associates (WJE) in Princeton, New Jersey. He specializes in the investigation and repair of the building enclosure, as well as peer review and consulting for new enclosure design. He can be reached at dpatterson@wje.com.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.
