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Plenum enclosures shall be constructed of materials permitted for the construction classification of the building.
Gypsum boards that form plenums shall be limited to return air systems if the air temperature does not exceed 125°F (52°C) and the building and mechanical systems' design conditions are such that the gypsum board surface temperature will be maintained above the airstream dew-point temperature. Gypsum return air plenums shall not be incorporated in air-handling systems that use evaporative coolers, or in mechanical equipment rooms.