All drywall is naturally fire-resistant. This building material was developed in 1916 by the U.S. Gypsum Company and is composed of a layer of gypsum plaster compressed between two sheets of heavy paper. Gypsum is a natural material with a high moisture content. When exposed to fire, the moisture in gypsum becomes steam, which helps suppress flames. Two special types of drywall, X and C, are especially fire-resistant and are called fire-rated drywall. Both have fiberglass added to the gypsum core; C has more fiberglass content and also a form of vermiculite, which stabilizes the gypsum core under heat.
Double the number of Type X sheets on the wall for a simple system installed over fire-resistant fiberglass insulation; each sheet of 5/8-inch Type X drywall is rated to resist fire for one hour, so two layers will produce a two-hour firewall. Put two sheets together on one side or use one sheet on each side of a wall. Do this only in situations where the added material will not affect other construction.
Use thicker drywall for a two-hour rating, if double-sheeting is not an option, with fire-resistant insulation between the wall studs. Cover wood-framed walls with a single layer of 3/4-inch Type X drywall rated for 120 minutes of fire resistance. Make sure the drywall is approved under ASTM standard E 119; check the markings on the drywall or ask the supplier if it meets that standard.
Build a two-hour exterior wall with steel studs, rather than wood, and special metal stud fiberglass insulation between the studs. Install 5/8-inch Type C drywall on the interior surface and 1/2-inch gypsum sheathing on the exterior. Finish the wall with a 1-inch coat of Portland cement gypsum plaster.
Use steel studs and two layers of 1/2-inch Type C drywall on each side of the studs for a two-hour fire rating on an interior wall. Mount a 1 1/2-inch fiberglass barrier inside the studs. Use special fiberglass rated for use in firewalls. The American Society for Testing and Materials and Underwriters Laboratory both have specifications for these materials.
Consider special wall assemblies from manufacturers who supply wall components that combine Type C drywall with special insulation for fire resistance and soundproofing. Some assemblies use three or four layers of Type X drywall in a double-stud configuration, so each wall segment has two vertical studs rather than one.