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By Jay Fox
With wildfires becoming more frequent and destructive across North America, designers are under increasing pressure to create more resilient building envelopes. In our latest Component Spotlight, Cascadia Windows & Doors’ Technical Director Michael Bousfield took us deep into the science of wildfire-resistant windows, examining how different glazing, framing, and materials options can make or break a building’s survivability in the event of a wildfire.
Estimates place one in six American homes within an area of significant risk of wildfire. This figure is increasing for several reasons. First, hotter and drier conditions are leading to a growing number of fires throughout North America. Second, urban sprawl and a growing population are leading more individuals to build in areas that are designated as Wildlife Urban Interface (WUI) zones. These are spaces where wilderness meets the built environment and where wildfire risk is often extremely high. This is particularly true in Western states like California. Finally, as more development expands into historically wild areas with dense vegetation, this increases the potential sources of accidental ignition either from infrastructure (e.g., downed power lines) or human error (e.g., forgetting to put out a backyard fire).
According to a September 2025 report from Realtor.com, “Approximately 5.6% of homes (worth $3.2 trillion) in the United States face severe or extreme risk of fire damage [emphasis added], and nearly 39% of these high-risk homes (worth $1.8 trillion) are in California.”
To adapt, research has shown that building owners need to take proactive steps that include:
Fire smart landscaping, which involves creating a defensible perimeter free of vegetation around any structure.
Fire-resistant building design, which relies on passive principles like improved airtightness and mechanical ventilation, as well as the elimination of complex geometries where embers can become embedded, smolder, and then ignite.
More fire-resilient and robust building materials.
It is rare for homes that are destroyed during wildfire events to be consumed by walls of flames that incinerate everything in their path. Instead, they are destroyed when nearby wildfires emit embers that become lodged in buildings’ nooks and crannies or when flaming debris falls near the exterior of the building. In the latter case, these fires tend to burn exceptionally hot but not for long. If the debris falls next to the building’s siding it will probably result in a partial burn. If it is next to a window, the heat can shatter the window, and then the curtains or furniture near the window can ignite. If the window frame is flammable, the flames can also use this point of failure to enter into the building.
This is why materials matter.
Cascadia has long championed fiberglass as a high-performance, durable, and low-carbon alternative to aluminum and uPVC. But as Bousfield noted, fiberglass’ thermoset resin structure gives it an additional superpower: it doesn’t melt or deform under heat, a key advantage when embers or radiant heat threaten a building’s envelope.
To confirm that Cascadia’s fiberglass-framed and multipaned windows could stand the heat, the company decided to perform a real-world test based on California’s State Fire Marshal (SFM) Standard Test Method 12-7A-2. Despite the rising danger of wildfires and the knowledge that windows are oftentimes a key weakness in building envelopes, there has been limited testing and guidance with respect to best practices for fire resiliency outside of California. Consequently, California’s test standard is regularly referenced in states and provinces outside of California and is as good a test as any to determine a windows’ level of resiliency during a wildfire.
During the test, Cascadia exposed their fiberglass windows to an eight-minute blast of direct flame, simulating the short, intense exposure a structure might experience when flaming debris lands at its base. The test is not meant to show that the window can endure the flames without damage. “The window’s job is to take that damage and protect the building behind it,” Bousfield explained.
Every prototype, including Cascadia’s baseline model, withstood the test, preventing flame penetration and protecting the building interior.
In addition to showing that Cascadia’s fiberglass windows can withstand the typical conditions that occur during a wildfire, the live-fire experiments revealed a few nuanced lessons about both glazing and frame design.
First, IGU choices matters. Thicker (≥6 mm) glass performed better, and tempering the inner pane proved critical to the survival of the window unit—and potentially the building. As Bousfield notes, the survival of the inner insulated glass unit is far more important than the outer one, which is likely to break even if treated. In fact, tempering the exterior IGU may make the window unit less resilient because tempered glass doesn’t just shatter; it tends to break off in large pieces that may allow heat to affect the interior IGU. Alternatively, annealed glass may also shatter but it is more likely to stay in place longer, acting as a temporary shield.
Second, frame geometry counts. Cascadia’s multi-chambered fiberglass frames charred on the surface but resisted distortion and retained integrity with or without mineral wool insulation. This was true after eight minutes and even after 20 minutes. As Bousfield noted, the frame “acts almost like a fire blanket to what’s behind it. Some heat transfers through, but not fire.”
Third, material choice is vital. Testing on vinyl frames showed that the material enters into what is known as its glass transition point around 180° F (82° C). When a material enters its glass transition point, it becomes rubbery and more malleable, at which point it can lose integrity and allow flames to penetrate into the building. Failure becomes far more likely at higher temperatures, as vinyl melts at 338-414° F (170-212° C) and ignites at 736° F (391° C). By contrast, fiberglass does not enter into a glass transition point or melt, and it only ignites at 830° F (433° C).
Taking what they learned from these tests, Cascadia has developed two wildfire-resistant variants of its Universal Series windows:
WFR (Wildfire Resistant)
WFR+ (Enhanced Wildfire Resistant)
The WFR meets the California test standard with minor upgrades and is designed to protect buildings that are within WUI zones at moderate risk of fire damage. The WFR+ model is equipped with an additional fire-retardant resin for even greater resistance and is designed to protect buildings that are in heavily wooded areas at greater risk of fire damage.
As Bousfield emphasized, the goal isn’t just compliance—it’s designing smarter envelopes that save structures and lives. “Windows are a specific vulnerability,” he said. “They can be the reason a building is lost—or the reason it survives.”
If you missed your chance to geek out on fire resilience life, you can watch the full recording on YouTube. You’ll gain practical insights on testing methods, frame material selection, and glazing strategies to enhance fire resilience—plus an inside look at Cascadia’s innovative research.
