The COVID-19 pandemic has spurred an existing trend for integrating the indoors and outdoors using arrays of multiple glass door panels. These reap the psychological benefits and energy savings of natural light and fresh air while still allowing closure for protection against the elements.

Such multi-panel exterior door systems often feature multiple sliding, operable panels that open sequentially to create a large wall opening. Transcending the 6- to 8-foot width of the traditional two-panel patio door is no longer reserved for fine resorts and commercial applications. In the residential sector, they are beginning to migrate from custom homes and resorts to tract communities, which will further spur demand.

For the first time, these large-format exterior doors are being recognized as a separate market segment in the FGIA 2019/2020 Study of the U.S. Market for Windows, Doors and Skylights, defining them as consisting of openings (larger than 8 feet by 8 feet) that can function either as entry doors or patio doors.

Aluminum and aluminum-clad wood doors represent approximately 60% of these products, while wood doors make up 18%. Other materials, such as steel, vinyl and fiberglass, comprise the remaining 22%. Vinyl and fiberglass are used most often in patio door applications, while steel door usage increases, particularly for very large doors.

Multi-slide configurations (using multiple tracks) comprised 35% of the 2019 U.S. market for large format doors while folding (hinged) versions made up 21%. Most large-format doors are within 10-20 feet long and therefore use anywhere from two to five panels. While most manufacturers can produce doors with as many as 10 to 12 panels, these are rarely specified.

To ensure long-term performance, large format door assemblies must be tested, rated and certified to meet requirements for basic performance attributes—structural integrity under wind loading, resistance to air infiltration and prevention of water penetration. Additionally, product-specific performance factors such as operating force (per ASTM E2068), forced entry resistance (per ASTM F842), thermoplastic corner weld strength, deglazing test (per ASTM E987), and life cycle testing (per AAMA 920 for AW-Class products only), must also be tested.

Generally, the North American Fenestration Standard (NAFS) limits products qualified for a specific performance class to products smaller than the frame size of the tested product. Applied to sliding doors, this means the door frame of the certified product cannot exceed the frame size of the tested product. This poses a potential problem for a multi-panel door in that testing, say, a 30-foot-long unit would be cost-prohibitive.

The FGIA Multi-Panel Door Testing for Certification Task Group has been focusing on a way to qualify multi-panel sliding doors based on the product’s panel size rather than the frame size. The goal is to develop a procedure to be added to NAFS that would allow manufacturers to avoid unreasonable testing expenses. The way folding doors are currently treated in NAFS offers a precedent for this methodology. Having surveyed manufacturers to identify and quantify key performance criteria versus the number of tracks in the door frame, the task group has determined that performance is determined by design, not the number of tracks.

In essence, a manufacturer would test a basic sliding glass door product in accordance with NAFS. The test sample would include all the design parameters and features to be included in the larger product line, such as astragals, interlocking panels, locking conditions, etc. Specific large-format sliding door compliance would then be based on extrapolating these test results via engineering analysis to a multi-panel system rather than being limited by frame size. Such an approach would qualify configurations with a greater number of panels, so long as the panels are not larger than those of the tested configuration and are constructed in the same manner.

Jason Seals is certification services manager for the Fenestration and Glazing Industry Alliance (FGIA).

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