Abstract: An air distribution unit for mounting in the ceiling of a room comprises a downwardly opening sheet metal box having a horizontal top wall and depending side walls. Conditioned air is introduced into the box through an inlet opening in the top wall and is diffused by a grille in the lower end of the box. A normally open damper automatically closes the inlet opening in the event of fire. The upper side of the top wall and the outer sides of the side walls are lined with panels of fire-resistant sheet rock which retards the transmission of heat through the box if a fire occurs.