FGIA Analysis May/June 2021

August 19th, 2021 by Nathan Hobbs

Sizing Up: Multi-Panel Doors Are Becoming a Unique Market Segment

By Jason Seals

The COVID-19 pandemic has spurred an existing trend for integrating the indoors and outdoors using arrays of multiple glass door panels. 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, these are 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.

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.

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%. The majority of large format doors are within 10-20 feet long and therefore use anywhere from two to five panels. While most manufacturers have the capability to 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, forced entry resistance, thermoplastic corner weld strength, deglazing test, and life cycle testing, 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 a 30-foot-long unit would be cost prohibitive.

Adjusting to the Times

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 currently are 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 in size 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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