If the glass in a window breaks during a wildfire, embers and flame can easily enter your home. Similarly, if your window frame ignites, it is possible that the resulting fire would burn through the frame material and ignite other material inside your home (for example, a curtain). Both of these scenarios could easily result in the loss of your home. Therefore, windows must be able to resist the following wildfire exposures:
- A radiant exposure severe enough to break the glass in your window or ignite the exterior siding directly below it. Burning vegetation could also ignite combustible siding.
- A flame contact exposure that could result from embers igniting vegetation and/or exterior cladding that burns up to your window.
The photograph below is from a laboratory test. Window failure occurred as a result of glass breakage. The exposure was flame contact from a propane gas burner located 2 feet below the window. The burner simulated flames from an ignited medium-sized plant.
Glass breakage occurs as a result of the temperature differences between the edge of the glass protected by the frame, and the glass exposed to the flame (the part of the glass you can see through). This non-uniform heating causes the glass to expand at different rates. Minor flaws at the edge of the glass start to grow, and if the temperature differences are large enough, the cracks grow and potentially break out.
Window failure can also occur if the frame materials ignited, with subsequent burn-through into what would be the living space in the house. Note than in the photograph shown below, flame penetration occurred at the horizontal separator (meeting rail) in this hung window. The exposure was flame contact from a propane gas burner located 2-feet below the window. The burner simulated flames from an ignited medium-sized plant.
Results from one study showed that for vinyl window frames, the horizontal separator shown in the photos above can be vulnerable to radiant heat exposures. At fairly low radiant exposures, the frame deformed, and the glass fell out. Results from testing done at the University of California (UC) did not show this effect. All of the double hung windows (i.e., windows where the upper and lower parts of the window can both move) were constructed with a metal bar in the separator. The bottom line of the UC research was that the glass is the most vulnerable component in a window and not the frame materials. This finding is also supported by research conducted in Australia.
Embers could land on a window sill, or as is shown in the photo below, the sill at an entry door. The embers could then ignite debris, or ignite the decayed trim. Decayed wood ignites as a lower temperature than that required for sound wood.
Burning embers could ignite this plant, which would then result in a flame impingement exposure of the window.
Because of the importance of glass in the performance of a window in a wildfire, the most important thing you can do is install dual-pane windows. With dual pane windows, the outer pane often serves as a thermal shield and protects the inner pane. The inner pane is allowed to heat up more slowly, and uniformly, and therefore may not fail even if the outer pane does. The frame material can be selected based on cost, aesthetics, energy efficiency, or other factors. If you purchase a window from a manufacturer that is a member of either the American Architectural Manufacturers Association (AAMA) or the Window and Door Manufacturing Association (WDMA), other window features that can improve the overall performance of your window will be present.
Tempered glass is stronger than annealed glass, and will provide additional protection during a wildfire. Tempered glass is more expensive, and will add approximately $1 per square foot to the cost of your window. Building Codes already require tempered glass in some locations, so some of your newer windows will already have tempered glass. For example, in newer construction, windows that come within 18 inches of the floor must have tempered glass. Sliding glass doors, and other doors with windows, and windows immediately adjacent to doors, will also have tempered glass.
A small white etching is often present in the corner of a piece of glass in a window if it is tempered. The California Building Code now requires at least one tempered pane in a dual pane IGU for new construction in wildfire prone areas.
Research has shown that insect screens improved the performance of glass under radiant exposures. Bronze, fiberglass (with polyvinyl chloride coating), and aluminum screens all improved glass performance (increased the time needed for edge cracks to develop). Results from this study showed that bronze screens were most effective, and aluminum the least effective. However, research at UC has shown that screens do not provide added protection from a flame contact exposure.
Shutters or Covers
For additional protection homeowners could consider taking additional precautions to protect your windows. These precautions include fabricating covers (for example, 1/2-inch plywood covers), cut to size and marked so that it can easily be installed over a window prior to evacuation. Shutters or other roll-down devices could also be installed. In this case, you will have to make these items part of your routine inspection and maintenance program to make sure they operate properly. All of these have the disadvantage of requiring an action to implement.
Some of the gels and foams marketed for structure protection during wildfires indicate they will also protect windows, but verification of these claims by an independent source isn't currently available.