For projects with typical opening sizes of traditional residential construction, many options for standalone rated windows exist. However, for the larger window sizes seen in today’s multifamily construction, few manufacturers offer a rated window product that can fill oversized openings without sub-framing. The cutoff for what constitutes as an “oversized opening” varies by product type and the manufacturer, but designers can identify these issues by reviewing the basis-of-design document to understand product constraints and avoid costly redesigns, testing fees, or change orders for additional framing members.
However, even in smaller openings, the question of whether to use sub-framing may still come up. Contractors sometimes propose the use of receptor frames in inconsistent existing openings to provide tolerance during window installation. Similarly, designers may specify them to accommodate movements such as slab deflection. Receptor frames capture the full perimeter of the window, leaving space for adjustment and movement. However, this creates yet another system of joints where water and air leakage can occur, unless adequately vetted beforehand and properly detailed.5,6 Identifying where additional installation and movement tolerances are needed in advance can help a design team identify these risks early and adjust the design to mitigate them.
Alternately, when openings become large or irregular enough, it is often simpler to use a storefront or curtain wall which can perform over larger opening sizes, rather than using a mulled window. Storefront and curtain wall frequently utilize starter sills and receptor frames to accommodate adjustment and movement, but these have a greater track record and higher availability of rated components. For this reason, designers targeting more reliable performance in large, punched openings may start by developing elevations and specifications that are more conducive to storefront and curtainwall systems.
AAMA 450
For mulled fenestration, the most crucial addition to a specifier’s toolbox is a performance specification that references AAMA-450, Performance Rating Method for Mulled Combination Assemblies, Composite Units, and Other Mulled Fenestration Systems. Although this industry standard for rating systems with sub-framing components has been in place since 2000, in the author’s experience, it was not widely embraced until a few years ago—since NAFS-17 lists it as a voluntary standard—and it did not address crucial detailing considerations that impact air and watertightness until its most recent update in 2020.
AAMA 450 is now quickly gaining traction in the industry, possibly due to the streamlined testing and submittal process it allows for. Previously, applicable mulled window test reports were often unavailable since testing all possible mulling configurations was often infeasible; however, AAMA 450 provides a standardized method for combining testing and structural analysis to certify product lines in a variety of configurations. This simplifies the process for manufacturers and gives specifiers both design flexibility and better performance documentation for the systems they select.
