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Philibuster: Mulling over mullions

May 4, 2023  By Phil Lewin

I was reminded recently of a project in an earlier part of my industry life when I was with a project installation company and me and George Torok, now chair of the CSA A440.4 committee on window installation, were doing a mock-up for a large group of townhouses. Most of the windows were straightforward individual units but the front of each home had a rather large combination assembly of two pictures over two sliding units, which we attempted to create with vinyl windows coupled together with the manufacturer’s basic coupler system. We’re talking about the 1990s, so “basic” was the only available coupler system. The combined unit would bend in and out when lightly pushed at the intersecting junction of all four window units and did not possess the stiffness required to resist design wind pressures. The only solution to solve the structural problem at the time was to go with aluminum windows and couple two full-height picture-over-slider windows. Mock-ups do save a lot of headaches!

This year, I received a call from a homeowner with the same issue: four separate windows, two pictures over two sliders, coupled vertically and horizontally. With the homeowner’s permission, I talked to the installation company, which was making every effort to successfully reinforce the combination unit’s mullions to be structurally sound. Once I was made aware of the brands the installation company was selling, I knew that one of them had a proper, tested system for coupling windows together and recommended they re-install the window using the tested coupling system.  But then I asked my colleague, Al Jaugelis at Fenestration Canada, about what kind of advice I could give to window manufacturers about this matter.

According to Al, here’s what the building code expects: that any window, whether an individual fixed, casement or sliding window, or a combination assembly composed of separate windows mulled together, must resist the wind and driving rain/wind pressures the code requires at the building’s location and the height of the window above grade. Those ratings can be determined using the Fenestration Canada “NAFS Performance Grade Calculator” on the Fenestration Canada website. The code expects that the windows delivered to any jobsite have labels showing they meet or exceed the required NAFS performance grade and water-penetration resistance test pressure at that location.

Let’s start with a building inspector coming to a home with one of those massive combination windows that are becoming ever-more common. If the inspector sees a label on each individual unit window that meets code requirements, is that enough? For many inspectors, it could be.


A well-informed inspector would be looking for something more: a single label for the overall size of the coupled window showing its overall rating, or labels that include both the individual window ratings as well as each coupling’s mullion assembly rating. The alternative is a single label for the entire combo unit that includes both the individual windows’ ratings as well as the coupling’s mullion assembly rating. Failing to report the mullion assembly rating means that, no matter how good and well-tested the individual units are, the combination as installed does not actually meet the building code.

So, what is a window manufacturer supposed to do? According to Al, discuss this matter with your test lab. Mullion assembly ratings can be determined by testing a coupled window or by using the methods in the AAMA 450 standard. I remember being faced with this issue years ago and sitting in meetings where we questioned whether every possible joint between two windows of each window style would have to be tested. Now that was a scary thought. Today, the test lab can help you determine the minimum testing required to qualify the several coupling options a manufacturer may have.

 For more information, download the “NAFS Labeling Guidelines for Canada” on the Fenestration Canada website.

Phil Lewin is technical director for SAWDAC.

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