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In a session sponsored by the of the Fenestration and Glazing Industry Alliance (FGIA), participants heard the latest research from an experimental campaign to investigate the performance of window components, particularly glazing types and frame materials, during a structure-to-structure fire spread scenario. engineers Daniel Gorham and Joseph Willi presented their findings along with Gavin Horn, Research Director, in a session at the FGIA Annual Conference entitled, “How Window Materials Perform Under Fire Exposure.”

Findings from this work may impact decisions of homeowners and homebuilders regarding the selection and installation of window assemblies in areas prone to wildland urban interface (WUI) fire hazards. Codes, standards and guidance documents provide requirements and recommendations for resisting exposures from WUI fires. Window performance has been shown to be a key factor in a structure’s survivability during exterior fire exposures, according to the Institute.

“Most of these fires start from an ember,” said Horn in his opening remarks. “They break off trees or shrubs or even other structures. If those embers find a receptive fuel bed, they can directly ignite structures.”

Horn said there is much to be learned from the aftermath of these events. “They have become all too common here on the West Coast,” he said. “We can view the chain of failure and retrace our steps. There are a lot of reasons why a structure might survive when one next to it did not.” He added that while construction plays a big role, so do factors like timing and magnitude of the wind. “We can take this information and help better prepare future homes, looking at individual components in a controlled manner.”

Building Susceptibility

Ignite mechanisms are exposure from flames and embers, explained Gorham. The magnitude and duration of that exposure is a function of fuel, distance between burning fuel and a structure or target and external factors like wind and other interventions. “Vulnerability can be mitigated by hardening structures for exposure,” he said. “The building envelope is crucial to this. A building is more than the sum of its parts: it is how those components interface with each other.” Components include the roof, eaves, siding, doors and windows.

“Doors can be resistant to embers, but we have to remember the human factor,” he said. “The best design can’t protect against the error of leaving a door open. When it comes to windows, install dual or multi-panel ones.”

Exterior Fire on Residential Windows

Willi went into the research itself, which studied structure-to-structure fire spread experiments. All consisted of a targeted façade wall with window pane assemblies and window frames made of both single materials (vinyl) and multiple materials (aluminum, fiberglass exterior, wood). “We also tested some window protections like shutters,” he added.

Window Pane Experiments

Four types of double pane window assemblies were studied: both panes annealed glass; both panes tempered glass; fire side annealed, back side tempered; and fire side tempered, back side annealed. Windows were mounted in the target wall facing the exposure. “Failure was defined as the first instance of cracking,” said Willi. “We compared heat levels at time of failure and looked at the number of total failures out of our 20 samples of all four assembly types.”

Key Takeaways

• Window pane assemblies with annealed glass on the fire (exterior) side and tempered glass on the back (interior) side outperformed windows with the opposite configuration.
• Window pane assemblies with both panes tempered glass performed the best of the four types considered in this study.
• Heat transfer through windows has the potential to ignite interior combustibles even without window failure.
• Frames and other window components should also be considered when addressing window failure from exterior fire exposure.

Window Frame Experiments

For the frame experiments, the team focused on vinyl, wood, fiberglass and aluminum frames. All windows were double pane tempered glass. They tested each frame type at four different positions in the façade, totaling 16 paired observations per frame for statistical comparison between types. “We compared the total number of occurrences, time relative to flashover and heat load,” said Willi. “Window events included back side smoke, front side fire, back side fire and sash failure.”

Window Protection Methods

Gorham shared some preliminary takeaways about protecting windows during fires but noted that their journal article is currently in peer review and is not final. One example was that oil-faced fiberglass was most effective, followed by the fiber cement board and intumescent wrap.

Gorham shared that, overall, FSRI has conducted more than 50 building-to-building fire spread experiments, leading to open access publications and informing many codes and standards. Read more about their work on .

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