Both surfaces of a two-hour-rated mass timber wall that serves as a required separation between...

The minimum additional protection is 1/2-inch gypsum board or an approved equivalent thermal barrier , making option C correct.

The wall in the question functions as an occupancy separation. Under IBC Section 508.4.4.1, mass timber elements serving as fire barriers or horizontal assemblies separating occupancies in Type IV-B or IV-C construction must be separated from the building interior by an approved thermal barrier consisting of gypsum board at least 1/2 inch thick , or a material satisfying both applicable NFPA 275 thermal-barrier tests.

Because the wall separates two interior occupancies, each face is exposed to interior space; consequently, the thermal-barrier requirement applies to both surfaces .

This requirement must not be confused with the wall ' s two-hour fire-resistance rating. The underlying mass timber wall must independently satisfy the required two-hour assembly performance. The 1/2-inch gypsum protection is an additional thermal-barrier condition imposed because the rated separation is constructed of mass timber in Type IV-C.

Option D would provide substantially more gypsum protection than the specific occupancy-separation provision requires.

References/Topics: 2021 IBC Sections 508.4.4 and 508.4.4.1; Type IV-B/IV-C mass timber occupancy separations; approved thermal barriers; NFPA 275.