August 8, 2019
Deborah Slaton and David S. Patterson, AIA
Selection of fenestration assemblies (utilizing both window and storefront systems) can be challenging due to the number of ancillary components/accessories available—materials with different performance characteristics compared to the main fenestration element. For example, aluminum windows (and storefront systems) are often paired with extruded aluminum subsill and receptor assemblies to better accommodate the installed condition. However, while these accessories are, in many cases, offered by manufacturers as part of a system or assembly, they are typically independent components with unique characteristics, such as resistance to water penetration and thermal efficiency, many of which may not align with the performance expectation of the overall assembly.
To highlight the complexities involved with such selections, in the example illustrated, a punched window for a high-rise, commercial building was detailed with a subsill component. The window unit was specified as an architectural grade window with an AW60 performance class rating—a specific window was listed as the basis of design and a subsill was specified as an accessory component (although no subsill performance criteria were mentioned in the specification). The document also referenced the American Architectural Manufacturers Association (AAMA)/Window and Door Manufacturers Association (WDMA)/Canadian Standards Association (CSA) 101/1.S.2/A440-17, North American Fenestration Standard/Specification for Windows, Doors, and Skylights, and AAMA 502, Voluntary Specification for Field Testing Newly Installed Fenestration Products.
The manufacturer of the window specified as the basis of design marketed the product as having a water-resistance test pressure (WRTP) rating of 720 Pa (15 psf). Although a minimum WRTP was unspecified for the project, AAMA/WDMA/CSA 101/1.S.2/A440 requires it to be 20 percent of the design pressure for an AW Class window. The WRTP for an AW60 window would be 580 Pa (12 psf). Although the subsill paired with this window was offered as an accessory by the manufacturer, the former had a WRTP rating of 360 Pa (7.5 psf) due to its limited leg height (a fine point unpublished by the manufacturer). Therefore, WRTP of the window assembly (window with subsill) was limited to the lowest-performing component of the assembly, the subsill, at 360 Pa. This detail was missed by the project team until leakage occurred during field testing of the assembly at 480 Pa (10 psf), or two-thirds of the 720 Pa at which the window was rated, as recommended by AAMA 502. After testing, the manufacturer indicated the cited WRTP of 720 Pa only applied to the window, and not to assemblies using the subsill on offer. The manufacturer also acknowledged the WRTP as an assembly was limited to the subsill. Following that logic, the manufacturer suggested the anticipated water resistance of the installed assembly should be two-thirds the rating of the subsill, or 240 Pa (5 psf), as the subsill governed WRTP. Thus, although the architect (and building owner) thought they were getting a high-performance window—rated at 720 Pa—for water penetration resistance, (or at least 480 Pa invoking the AAMA two-thirds rule), following the manufacturer’s rationale the assembly should only have to meet a WRTP of 240 Pa when installed. One can imagine the conversation that ensued.
This example highlights the importance of understanding the performance capabilities (and limitations) of all components/accessories of a fenestration assembly during the selection process, and clearly outlining expectations for these systems in the specification—expectations that should be confirmed with product manufacturers when specifying non-custom assemblies.
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 email@example.com.
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 firstname.lastname@example.org.
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