Wildfire-Resistant Windows: What the Codes Require

In wildfire areas, building codes ask that windows hold together long enough under heat to keep flames and embers out of the house. The 2024 International Wildland-Urban Interface Code accepts tempered glass, multilayered glazed panels, glass block, or glazing with at least a 20-minute fire protection rating in its two stricter construction classes. California’s code, in force since January 1, 2026, requires multipane glazing with at least one tempered pane or one of three alternatives.

How windows fail in a wildfire

Windows fail when heat or flying debris breaks the glass, or when a frame deforms and opens gaps; either way, flames and embers can get in. The University of California’s fire extension program (UC ANR) describes each route:

• Heat. Glass can break when exposed to radiant heat or direct flame, and the length of the exposure matters. UC ANR documents an outer pane that broke under radiant heat from a neighbor’s burning house.
• Debris. Wind-blown debris can break glass during a fire.
• Frames. In vinyl windows, gaps can open between the frame’s interlocking members and the glass at low radiant exposure, letting in embers or flame. UC ANR reports that studies found this deformation does not occur when the member has an aluminum reinforcement inside.
• Glass type. A multi-pane window performs significantly better with tempered glass, which is much more resistant than regular annealed glass. UC ANR notes that even one tempered pane can improve fire resistance.

What the International WUI Code requires

The 2024 International Wildland-Urban Interface Code (IWUIC) sets glazing requirements for its Class 1 and Class 2 construction and none for Class 3. It is a model code and applies only where a state or local government adopts it. It works in three steps:

1. Scope. Buildings “hereafter constructed, modified or relocated into or within” wildland-urban interface areas must meet the construction requirements of Chapter 5 (Section 503.1).
2. Class. A table assigns Class 1, 2 or 3 ignition-resistant construction based on the fire hazard severity of the site, the defensible space around the building and the water supply (Table 503.1).
3. Glazing. For Class 1 and Class 2, exterior windows, window walls, glazed doors, windows within exterior doors and skylights must be tempered glass, multilayered glazed panels, glass block, or have a fire protection rating of at least 20 minutes (Sections 504.8 and 505.8). Class 3 construction has no glazing requirement.

The class, and whether the code has been adopted at all, is decided by the local jurisdiction and the site.

California’s rules since January 1, 2026

California reorganized its wildfire building rules in the 2025 edition of its building standards code. The California Building Standards Commission says the 2025 edition was published July 1, 2025, and took effect January 1, 2026. It created a new California Wildland-Urban Interface Code, Title 24, Part 7, based on the IWUIC. The code moved in provisions formerly found in Chapter 7A of the California Building Code, Section R337 of the California Residential Code and Chapter 49 of the California Fire Code.

Section 504.8 of the new code requires exterior windows, window walls, glazed doors, windows within exterior doors and skylights to be one of:

1. multilayered glazed panels with at least one pane of tempered glass;
2. glass block;
3. glazing with a fire protection rating of at least 20 minutes, tested to NFPA 257 or UL 9;
4. glazing that meets the performance requirements of State Fire Marshal Standard 12-7A-2.

Like the model code, it applies to buildings “hereafter constructed, modified or relocated into or within” wildland-urban interface areas (Section 503.1). Guidance written before 2026 cites Chapter 7A or R337; in the 2025 codes those provisions are in Part 7.

Section 503.1 also lists exceptions, and one matters for older houses: additions to and remodels of buildings originally constructed before July 1, 2008 are exempt. Whether replacing windows in an existing house counts as a modification at a given address, and whether that exception applies, is a question for the local building department. Ask before ordering.

Guidance beyond the code minimums

FEMA and UC ANR recommend double-pane windows with at least one tempered pane, metal or reinforced frames, fine metal or fiberglass screens, cleared surroundings, and closing everything up before leaving.

FEMA’s April 2025 advisory, written after the Maui wildfires, recommends:

• replacing single-pane windows and glass doors with double-pane glass, preferably with at least one tempered pane;
• metal window framing, with fiberglass or metal screening of no more than 1/8-inch mesh;
• replacing plastic skylights with tempered glass in noncombustible frames;
• closing windows, doors, skylights and garage doors before leaving home on Red Flag Warning days or under evacuation orders.

UC ANR adds:

• Clear the area. Keep vegetation and combustible materials away from windows to reduce long radiant heat exposure and ember entry.
• Check vinyl frames. Confirm that vinyl windows have a vertical or horizontal reinforcement bar.
• Consider shutters. They add some protection against debris and radiant heat, especially if the house is within 30 feet of a neighbor’s home or building.
• Close up before leaving. Close all windows and skylights before evacuating, and cover windows with plywood if there is time.
• Identify tempered glass. Look for a small label in a corner of the window.

FEMA also stresses defensible space, an area of limited combustibles that extends up to 100 feet from the house, and recommends structural hardening where neighbors are closer than that.

Finding out whether your address is in a risk area

Check exposure on the USDA Forest Service’s risk site, and ask the building department which code applies; they are two separate questions.

• Exposure. The USDA Forest Service’s Wildfire Risk to Communities site shows long-term wildfire risk by community. It was created under the direction of Congress, first launched in 2020 and completely updated in May 2024. It shows long-term risk, not current fire conditions.
• Code. The applicable code comes from the building department. In California, the relevant maps are Fire Hazard Severity Zones and designated wildland-urban interface areas, which the new code’s Chapter 5 uses to set construction standards.

When to bring in a professional, and what to ask

A window replacement in a wildfire area is a code question as much as a product choice, and the answers differ by address. Questions for the building department:

• Is this address in a wildland-urban interface area or Fire Hazard Severity Zone, and which code edition applies?
• Does replacing existing windows trigger the wildfire glazing requirement here, and does any exception for older buildings apply?
• Under the IWUIC, which ignition-resistant construction class applies to this site?

Questions for a window contractor:

• Which of the code’s glazing options does the proposed window meet, and is at least one pane of each unit tempered?
• For vinyl units: do the frames and interlocks have aluminum reinforcement?
• What screens come with the windows, and what is the mesh size?
• For skylights: is the glazing tempered glass rather than plastic?
• Will the tempered panes carry permanent marks that an inspector can read?

Keep the product documentation. If a building department or insurer asks what was installed, the labels and invoices answer it.