Providing a fire-rated wall may not be common in single-family housing, but knowing when such an assembly is required by code–and how to comply–can be a consulting opportunity that builds customer loyalty. It's likely that everything you need is already in stock.
According to the International Residential Code (IRC), specifically table R-302.1, a one-hour fire-rated, load-bearing wall–defined as one that will not collapse or transmit flame or high temperature for one hour after a fire commences–is required when two detached dwellings are within 5 feet (wall to wall) of each other.
For attached homes, from townhouses and duplexes to apartment buildings, all shared walls must meet that standard or better depending on conditions and local fire and building code interpretation and enforcement. Some codes and jurisdictions require up to four-hour fire wall assemblies.
A one-hour fire-rated wall assembly consists primarily of wood, light-gauge steel framing, or concrete block, insulation, and gypsum wallboard. How it is spec'd and assembled determines how well those materials delay the spread of fire. Fortunately, the IRC provides several avenues to compliance, allowing builders to meet the code with materials and methods that suit their culture, climate, and budget.
"There is no 'best' solution" among the dozens that comply with the code, says B.J. Yeh, director of the Technical Services Division at APA-The Engineered Wood Association in Tacoma, Wash. "It's what is most economical and comfortable for the builder."
When properly assembled with other, less-flammable materials, a wood-framed wall can achieve some measure of fire resistance, giving occupants and emergency crews time to find the fire, contain or suppress it, and evacuate the building before it collapses.
A Common Assembly
Among options allowed by code, a common load-bearing exterior wall assembly, as tested and approved by Universal Laboratories (U.L.), includes standard 2x4 wall studs placed 16 inches on center and the cavities filled with a minimum of R-13 glass fiber insulation faced with Kraft paper or a vapor-retarding foil.
On the inside of the studs is a sheet of 5/8-inch thick, Type X gypsum wallboard hung vertically to reduce the number of joints–and thus potential points of fire entry–along the height of the wall. Type X wallboard includes special fire-resistant additives to a gypsum core that's already 21% water, which under high heat releases as steam to help slow the spread (and heat) of the fire.
In addition, this assembly calls for a minimum 15/32-inch sheet of APA-rated Exposure 1 plywood or oriented strand board sheathing, followed by another layer of 5/8-inch Type X gypsum wallboard, on which an air/vapor barrier (housewrap or other membrane) can be applied before installing the finish cladding.
A critical, if often overlooked, component of a fire-rated wall assembly is the fasteners that tie the various layers together to maintain proper structural integrity. Specific to the outside gypsum panel in this assembly, the IRC requires 2-1/4-inch, #6 Type S drywall screws at 12 inches on center throughout the panel, drilled through the plywood or OSB layer and, when possible, into the wall framing.
Generally, the other components of a fire-rated wall require longer screws or nails, perhaps up to 1-1/4-inches long, to fasten the other panels and finishes. Alternatives that achieve the same rating include boosting the thickness of the wood sheathing to eliminate the gypsum layer on the outside, and using metal studs or lightweight concrete masonry units (CMUs or block) for the wall structure.
The key to meeting code and providing adequate fire resistance, says Yeh, is to seek out visual references to the code language and follow directions to the letter.