Is Drywall Fire Resistant?
Manufacturing processes increase the fire resistance of drywall. (Photo courtesy of Angie's List member Robert G. of Carlsbad, Calif.)
Drywall is relatively fire resistant. It's composed of gypsum pressed between two sheets of thick paper. Gypsum is a soft mineral that by itself is not flammable. Additionally, water is incorporated into its structure. In the presence of fire, heat energy vaporizes the water, removing it gradually. Water effectively blocks the transfer of heat through the drywall. Once the water dissipates, the gypsum begins to heat past the boiling point of water, and may begin to burn.
Of course, fire resistance includes not only the products’ ability to resist ignition but also to maintain structural integrity. Some drywall manufacturers add glass fibers to the gypsum, which increases fire resistance of drywall. The glass fibers are non-combustible and help maintain the integrity of the drywall as it is dehydrated.
Type X designated gypsum boards meet the American Society for Testing and Materials (ASTM) guideline that 5/8-inch sheets applied in a single layer to both sides of standard wood framing will resist penetration by fire for at least an hour. Half-inch sheets applied in the same manner can resist fire for 45 minutes.
The fire resistance of drywall is linked not only to the way that it is manufactured, but also to the way that it is used in construction of walls and ceilings. Local drywall contractors will know local and state standards and fire codes in regards to drywall application and finishing.
Once drywall is attached to either wood or steel studs, small gaps between the sheets would allow air, smoke, and fire to pass through. To close the joints of adjacent drywall boards, a drywall contractor will apply tape and joint compound. Drywall may be applied in multiple layers to increase fire resistance. To increase cost effectiveness, a drywall contractor will use fire-resistant drywall in rooms where fires are likely to originate, such as kitchens and garages.
Fire-resistant drywall can't guarantee that it will always stop a fire from spreading. The goal in using fire-resistant drywall is to slow the spread of fire from its origin, limit the damage that is done, and provide occupants time to evacuate to safety. The inherent non-combustibility of drywall, appropriate selection of drywall type, and proper installation all contribute to the overall resistance of drywall to fire.
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Editor's note: This is an updated version of an article originally posted on Dec. 15, 2011.