In a 3300sf+, type VB, 1 story, business use building the plenum enclosure is designed to be the inside face of the exterior walls, the topside of gyp board or acoustical tile ceiling and the bottom side of a flat roof (exposed metal deck).
1. The exterior wall assembly on the inside face would be foil faced insulation, metal studs 16” oc and the backside of wood plywood sheathing. Since this is “enclosing” the plenum and the building construction type is VB these “enclosing” wall materials, specifically the plywood sheathing, does not need to have a flame spread or smoke development rating to comply right?
2. If an interior metal stud wall goes up through (“into”) the plenum to the bottom of the deck these materials would have to be flame spread 25 or less/smoke developed index 50 or less OR per exception 5, IMC section 602.2.1 be “enclosed” with gyp bd or other approved material because it's "within" the plenum correct?