Commonly, ceiling mounted air diffusing devices, called diffusers, are located adjacent the discharge ends of ventilation ducts in a building. These devices direct forced hot or cold air outwardly generally parallel to the ceiling surface. Panels of the ceiling located adjacent the diffusers often become soiled with sir born particulates, dust and dirt moving with the air flowing from the diffusers. Periodic cleaning and eventual replacement of the panels is required. Also, hot air flowing along the ceiling does not circulate downwardly to heat the space above the floor in a heat efficient manner.