The use of gypsum material (drywall) for the construction of a return air duct system or plenum is allowed, provided that the air temperature does not exceed.......... Gypsum board is a composite material commonly used to build air plenums, shafts, and spaces. Air temperatures greater than 125℉ will, over time, dry out the paper facing material on the drywall and will lead to deterioration of the panel. Drywall can also deteriorate when exposed to moisture or condensation. For this reason, drywall must not be used with evaporative cooling equipment (or swamp coolers).