Upgrade to Heat and Flame-Resistant Windows

05/03/2023

Why do they matter?

Heat from a wildfire can fracture glass and cause it to fall out, potentially allowing flames and firebrands to enter your home. If an opening is breached and embers enter your home, fire can spread rapidly.

What do I need to know?

• The glazing (glass) in windows is vulnerable to fracture due to heat from a wildfire, as well as breakage from windborne firebrands.
• However, single or double pane tempered glass windows, doors, and skylights typically fracture at higher exposures, well above the radiant heat exposures capable of igniting the surrounding wood.
• Window, sliding glass door, and skylight frames are made of metal, plastic, wood, or a combination of these materials. Plastic and wooden frames may burn or melt. The failure of the frame or sash may mean that all the glazing falls out.
• Skylights on steep-sloped roofs can experience the same heat\temperature exposure as a window in a vertical wall.

Where do I start?

• Use multi-pane windows or tempered safety glass.
• Fireproof shutters can protect large windows and glass doors from radiant heat.
• Consider the size and materials for windows: smaller panes hold up better than larger ones; double pane glass and tempered glass are more effective than single pane glass; and plastic skylights can melt.
• Recommended glass products for homes in wildfire zones are laminated glass, tempered glass, glass with a low emissivity (low-e) coating, fiberglass-reinforced translucent glazing, and insulated glazing units (IGUs). Annealed glass, ceramic glass, and plastic glazing are not recommended.
• An insulated glazing unit (IGU) consists of two or three panes of glass that are separated by a sealed air space.
• There are many window and skylight frame products. Use metal or metal-clad wood, not wooden and plastic frames, to avoid window failure.

FEMA P-737, Home Builders’ Guide to Construction in Wildfire Zones provides the following guidance on recommended glass and types of glass to avoid:

Recommended glazing types

Laminated glass. Laminated glass provides resistance to windborne firebrands. If a firebrand strikes with enough momentum to break the glass, the plastic film in the core of the glass will keep the glazing in the frame, allowing the broken glass to continue to resist firebrand impacts, embers, and hot gases. If the plastic film in the core gets sufficiently hot, the pane will delaminate whether or not the glass has been broken. If laminated glass is specified, it should either be protected by shutters, as discussed below, or combined with tempered glass in an IGU.

Tempered glass. Tempered glass is more resistant to heat and flames than laminated glass or annealed glass. The resistance of tempered glass can be enhanced with a low-e coating or a proprietary reflective coating. Firebrands with sufficient momentum can break tempered glass. To avoid breakage, the glass can be protected by shutters. Another alternative is to specify and install an IGU with a laminated glass inner pane.

Low-emissivity (low-e) coating. Glass with a low-e coating provides a higher level of resistance to radiant heat than other types of glazing because the coating reflects radiant heat, reducing the probability that the heat will be able to enter the building. The coating should be on the inner surface of the exterior pane.

Proprietary fiberglass-reinforced translucent glazing. This product is available for skylights and walls. The skylight material has a Class A rating. See Fact Sheet #5, Roofs, for a discussion of this type of rating.

Insulated glazing unit. An IGU consists of two or three panes of glass that are separated by a sealed air space. Double-paned annealed units last about 10 minutes in a wildfire, twice as long as single-paned windows. In many cases, 10 minutes is long enough to provide protection from the fire. If the first pane fails, the second pane must still be penetrated. Laminated glass, tempered glass, and glass with a low-e coating can be combined in various ways into an IGU.

NOT Recommended

Annealed glass. Annealed glass (also known as common float glass) is commonly used in residential windows. Annealed glass is the most susceptible to wildfires of the various glass types and is not recommended for homes in wildfire zones unless protected by shutters.

Ceramic glass. This specialty glass is effective at resisting flames, but it transmits radiant heat readily. If ceramic glass is used for exterior glazing, heat that is high enough to cause ignition can be transmitted into the interior of the building. Ceramic glass is not recommended for homes in wildfire zones.

Plastic glazing. Acrylic and polycarbonate are often used in skylights and sometimes in windows. Because plastic glazing can melt during a wildfire, it is not recommended for homes in wildfire zones.

Discover technical details:

In fire-rated walls, use windows and sliding glass doors that are commensurate with the fire rating of the wall. For example, a 1½-hour rated window is intended to be used in a wall with a 2-hour rating, and a 3/4-hour rated door is intended to be used in a 1-hour rated wall. However, you can use a window with a higher fire rating. If a fire-rated wall is not specified, use an IGU with a metal or metal-clad wooden frame.

More Resources:

• FEMA P-737, Home Builders’ Guide to Construction in Wildfire Zones.
• FEMA. How to Prepare for a Wildfire.
• FEMA. Rebuilding After a Wildfire.