Final answer:
The maximum perm rating allowed for an exterior wall assembly is 5 perms.
Step-by-step explanation:
The maximum perm rating allowed for an exterior wall assembly is b. 5 perms. Perm rating measures the rate at which moisture can pass through a material. A lower perm rating indicates a greater resistance to moisture vapor transmission, which is important for preventing moisture damage in an exterior wall assembly. That's why the maximum perm rating allowed for an exterior wall assembly is b. 5 perms.