In this section we will discuss the performance of windows. This would include the framing material and glass.
Glass Windows: Potential Wildfire Intrusion
If the glass in a window breaks during a wildfire the fire can easily enter your home. Similarly, if your window frame ignites, it is possible that the fire could burn through the frame material, and ignite other material inside your home. 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. Burning vegetation could also ignite combustible siding.
- A flame impingement exposure that would result from embers igniting vegetation and/or exterior cladding that burns up to your window.
- The impact from burning embers on the glass. Remember that during a wildfire, your home can be subjected to exposure from thousands of embers for hours before and after the relatively short time (minutes) it takes for the wildfire to actually pass by your home. Embers could also land on the window sill and ignite debris that has accumulated.
In this laboratory test, window failure occurred as a result of glass breakage. The exposure was flame impingement from a propane gas burner located 2 feet below the window. The burner simulated a medium-sized plant on fire.
Glass breakage in a window occurred as a result of 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). These temperature differences cause the heated 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 glass will crack, grow, and potentially break out.
In this laboratory test, window failure occurred as a result of the ignition of the frame material, with subsequent burn-through into what would be the living space in the house. Note than in this case, flame penetration occurred at the horizontal separator in a hung window. The exposure was flame impingement from a propane gas burner located 2-feet below the window. The burner simulated a medium-sized plant on fire.
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 Forest Products Laboratory (UCFPL) 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 an aluminum bar in the separator. This aluminum reinforcement is usually present because it is needed to resist wind loads. The bottom line of the UCFPL research was that by far the important factor in determining the performance of windows under wildfire exposures is the glass, and not the frame material. This finding is also supported by research conducted in Australia.
Burning embers could land on a window sill, or as is shown in this photo, 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.
Windows: Possible Solutions
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 though the outer pane does. You can select the frame material that makes the most sense for you, based on cost, aesthetics, energy efficiency, or other issues that are important to you. 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.
Above is a cut-away view of two panes of glass in an insulating glass unit (IGU). Because of energy code requirements, most windows in new construction consist of two or more pieces of glass in an IGU.
Tempered glass is stronger than 'regular' annealed glass, and will provide additional protection during a wildfire, but we think dual pane is the most important part of the equation. Tempered glass is also 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 may 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. Since it is small, it may be difficult to read.
Low-E coatings have sometimes been discussed as a means of enhancing the wildfire performance of your windows. Low-E coatings are always on one of the inner surfaces of the IGU, either on the pane closest to the outside, or the one closest to the inside. If it's on the outer pane (i.e., the inner surface of the outside pane), it could reduce the performance of the glass because the exposed glass would heat up more quickly relative to the glass protected by the frame.
Insect Screens
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 UCFPL has shown that screens do not provide added protection in a flame impingement exposure.
Unfortunately, in a severe firestorm, your windows will still be vulnerable. Both panes are likely to break and fall out. Glass that would perform better (e.g., laminated glass) is currently very expensive, and therefore not affordable for many homeowners.
For additional protection homeowners should consider taking additional precautions to protect your windows. These precautions include fabricating covers (for example, 3/4-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.