A.Lynn Underwood, an engineer, licensed contractor, and building code official in Norfolk, Va., responds: There are two approaches to code compliance: prescriptive and performance. The prescriptive approach is like reading a cookbook: Follow the recipe spelled out in the code book — in this case, the 2006 IRC — and you won't have any trouble with your local code official. Among other conditions, Section R309.2 of the IRC requires that "garages beneath habitable rooms shall be separated from all habitable rooms above by not less than 5/8 inch (15.9 mm) Type X gypsum board or equivalent."
The performance approach to code compliance is trickier. In the code language above, the key word is "equivalent," which means that materials that can be shown to perform equal to or better than 5/8-inch Type X gypsum board may also be allowed. For example, according to Table 720.1(3) in the IBC (items 21 and 22), two layers of 5/8-inch Type X...
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