AskCodeMan

Hi Jerry,

Builder use rigid duct board instead of drywall inside the ac closet wall, is this allowed?

Hi Richard,

Presumably for the return air plenum - duct board is a lot better than drywall. While drywall "is allowed", duct board "is preferred".

The key being that the duct board is used for the return air plenum, not for the walls of the closet. The walls of the closet will become "concealed space" and would need gypsum board to be installed on the studs to serve of a wall enclosure to allow proper draftstopping and/or fireblocking, the duct board would then be inside the closet walls acting as a return air plenum or a supply air plenum.