How to Keep the Fire Out: Fire-Resistant Exterior Wall Components

04/14/2023

Why do they matter?

Exterior walls can ignite from a wildfire’s radiant and convective heat. A fire on an exterior wall of your home can ‘bridge’ to more vulnerable areas such as eaves, soffits, vents, and windows. Protect your home by making your exterior walls fire resistant.

What do I need to know?

• Cement, plaster, stucco, and concrete masonry such as stone, brick, or block resist heat and flames. These exterior components are known as the “cladding” of the building.
• Some cladding materials will not burn, but they may lose their integrity when exposed to high temperatures and fall away or melt, allowing fire a direct path inside the home (ex: vinyl siding).
• Windborne embers and firebrands may spread fire as well. Embers can become trapped in cracks in walls, window openings, and door trim boards and ignite combustible materials. Windborne firebrands can ignite wall coverings.
• Exterior wall fire resistance is dependent mostly on the construction materials and the amount of nearby combustible material.

Where do I start?

New Construction

• Choose exterior wall coverings that are noncombustible or fire-resistant and not susceptible to melting.
  • Recommended materials: Concrete, fiber-cement panels or siding, exterior fire-retardant-treated wood siding or panels, stucco, masonry, and metal.
    • These coverings should not ignite and fuel the fire.
    • Insulated concrete form (ICF) walls provide exceptional fire resistance as well as cast-in-place concrete or fully grouted concrete masonry units. If you choose ICF, use a stucco or masonry wall covering to protect the plastic foam forms.
  • Not recommended materials: not fire-retardant-treated wood siding, vinyl siding, metal siding susceptible to warping, and an exterior insulation finish system.
• If fiber-cement or metal wall coverings are used, install one layer of 5/8-inch type X exterior gypsum board with taped joints underneath housewrap.
• For vinyl siding (vinyl siding is not recommended), install one layer of 5/8-inch type X exterior gypsum board with taped joints underneath housewrap.
• Use noncombustible or fire-resistant material such as exterior fire-retardant-treated wood or fiber-cement for trim boards around doors, windows, eaves, and corners.

Existing Buildings

• Replace exterior wall coverings that are combustible, are susceptible to melting, or can readily transmit heat (ex: not fire-retardant-treated wood siding, vinyl siding, metal siding, and an exterior insulation finish system) with one of the recommended coverings (concrete, fiber-cement panels or siding, exterior fire-retardant-treated wood siding or panels, stucco, masonry, and metal).
• Before you replace vinyl or metal siding, look for an underlying gypsum board substrate as that may mean you don’t need remedial work.
• Check to see if you can keep the existing covering and cover it with 5/8-inch type X gypsum board and a new covering.
• Maintain and remove combustible debris (such as vegetation and leaves) and firewood near the exterior walls regularly to reduce the potential of ignition during a wildfire.

More Resources:

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