Hi- IMC 602.2.1 says: Materials within plenums. Except as required by Sections 602.2.1.1 through 602.2.1 .6, materials within plenums shall be noncombustible or shall have a flame spread index of not more than 25 and a smoke-developed index of not more than 50 when tested in accordance with ASTM E 84 or UL 723. What are the FSI and SDI for 2x wood studs? Since 2x wood studs are allowed to be used as fire stop, can wood studs used for wall bracing, etc., be exposed in the return air plenum? Thank you.