Deborah Slaton and David S. Patterson, AIA
Selection of fenestration systems for specific wall assemblies is a process in which performance characteristics, such as structural capacity, the resistance to air infiltration, and water penetration, and thermal attributes are evaluated along with aesthetics and cost. However, the integration of the system(s) under consideration with the adjacent exterior wall assembly is often overlooked in the selection process.
Modern exterior wall construction typically includes an air and water barrier to resist the transport of exterior air and bulk water into the wall assembly. These barriers can be vapor permeable or non-permeable depending on the wall construction, building type, and climate zone, and are considered as the wet-dry line of the opaque wall assembly. A cavity (or air space) is often created to provide an unobstructed drainage path for bulk water entering the system outboard of the barrier, and/or ventilation of the interstitial ‘wet zone’ of the assembly. Therefore, for the air/water barrier (AWB) to be effective, its continuity at penetrations, such as at windows, doors, louvers, and utilities, must be maintained.
Once it is selected, the integration of the fenestration systems with the AWB should be clearly developed and detailed to illustrate the design intent, an often overlooked step, as the AWB (typically illustrated as a heavy line) is often simply shown terminating at a schematic profile of a window or door frame. This simplification of a potentially complex integration can result in confusion during bidding and unanticipated construction costs.
In the example shown, a storefront system is installed in exterior wall assemblies with different claddings (stucco and exterior insulation and finish system [EIFS]) but similar construction—each of the assemblies is constructed with a drainage plane exterior of the AWB. The storefront assembly consists of open-backed extruded aluminum framing members with a single perimeter weather seal positioned at the forward return leg of the extrusion, as is common with this system. In both examples, the storefront is positioned in the opening such that the perimeter seal is forward of the drainage plane of the wall assembly, which aligns with the interstitial air space between the wall construction and storefront—a space that communicates with the wall construction inboard of the AWB and with the building’s interior, as the AWB is not integrated with the storefront framing. Since integration of an AWB via a detail strip or transition membrane is impractical with the storefront system, introduction of a cavity closure is necessary to maintain the continuity of the AWB and prevent moist cavity air and bulk water within the drainage plane of the wall assembly from entering the building interior.
As illustrated, understanding the potential for integration of fenestration system(s) with the adjacent wall construction to maintain the continuity of the AWB is essential to obtaining a successful installation. The importance of proper integration of the air/water barrier of the exterior wall assemblies should be stressed in the specification sections for the AWB as well as for the fenestration system(s), and the intended integration should be clearly detailed on the drawings to avoid any misunderstanding during bidding and installation (Jeffrey N. Sutterlin, PE, of the Princeton office of Wiss, Janney, Elstner contributed to this article).
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 firstname.lastname@example.org.
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 email@example.com.