AVS Club Gold
Join Date: Jul 1999
Location: Near an airport
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Products used for interior sheathing must comply with a long list of IBC and IRC codes and be certifed for that specific use by ICC-ES, Architectural Testing CCRR or other accredited certification and testing labs. These would include, but not be limited to, racking and shearing testing (ASTM-E 72-02), seismic, wind loading, fastener tests (etc, etc, etc) and, in particular NFPA 286-06, ASTM-e 84 and others (Corner burn, flame spread, barrier, etc).
Drywall meets the class A rating for use as an interior sheathing when applied to the specifications in the testing report. Some drywall is specifically tested and certified for use as a fire barrier but must be specifically certified as a barrier meeting IBC requirements. Not all plywood sheathing will meet Class A requirements (none that I know of meet fire barrier requirements). I have seen some plywood meet the requirements when coated with an intumescent paint.
Check with your local authority to see if plywood is accepted in your area as an interior sheathing. In some cases it is, in other cases only when covered with drywall.