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Windows

Windows fail in a wildfire two ways: the glass breaks from heat, or the frame ignites or deforms. Either way, the opening lets embers into your home, where they ignite furnishings and burn the house from the inside. Upgrading glass is one of the highest-value hardening investments you can make.

How Windows Fail

An open window is the worst case, so closing windows if you have time before evacuating matters. Closed windows fail when radiant heat creates temperature differences between the exposed glass and the glass shielded by the frame. Those stresses grow the tiny cracks that exist at every glass edge until the pane breaks. Larger windows have more edge and more cracks, which makes them more vulnerable than small ones, and single-pane annealed glass is the most fragile common configuration.

The upgrade that matters: tempered, dual-pane glass. Tempered glass is about four times stronger than the annealed glass in most older windows and far more resistant to heat. California's building code requires at least one tempered pane in multipane units for new construction in fire hazard zones. When replacing windows, choose dual-pane units with tempered glass, and prioritize windows that face large areas of vegetation.

Glass Matters More Than Frame

Older guidance often warned against vinyl and wood frames. The research is more specific. Testing found glass is usually the most vulnerable component, and frame material can be selected on cost and aesthetics once the glass is right (McArthur 1991; UC Fire Research Laboratory). The vinyl-frame problem occurs in windows with unreinforced horizontal or vertical separators, which can deform under radiant heat and let the glass unit fall out. Vinyl windows certified to the AAMA/WDMA/CSA 101/I.S.2/A440 standard include internal reinforcement bars that prevent this, so look for that certification label when buying vinyl windows.

Coatings, Films, and Expensive Glass

  • Exterior reflective films have been shown to protect glass against radiant heat in Australian testing. They are sold mainly for energy savings; if you consider one, consult an energy professional about the other effects.
  • Laminated glass performed no better than ordinary annealed glass in the same testing.
  • Low-E coatings, applied to the inner surface of the outside pane of a dual-pane unit, are predicted by modeling to improve fire performance, though this has not been confirmed in laboratory tests.
  • Ceramic, borosilicate, and intumescent-filled glazing resist breakage well but cost more than most budgets allow. Automatic or operable shutters provide comparable protection against radiant heat for less, and they also protect against flame contact.

If your site could plausibly deliver those extreme exposures to a window, the better investment is upstream: fix the defensible space that would produce the exposure.

Window Screens

Screens help. Bronze, coated-fiberglass, and aluminum screens all delay glass edge-cracking under radiant heat, with bronze the most effective and aluminum the least. Screens do not protect against direct flame contact, such as burning vegetation under the window, which is one more reason nothing combustible belongs under a window. Fine window screening (about 1/16 inch) also strips energy from embers passing through it: if the glass fails but the screen holds, the embers that get in are small and unlikely to ignite furnishings. Keep easily ignited materials (curtains, papers, furniture) away from windows facing vegetation during fire season.

Adapted from Quarles et al., Home Survival in Wildfire-Prone Areas: Building Materials and Design Considerations (UC ANR Publication 8393, PDF). See also CAL FIRE: Hardening Your Home.