Ceiling Cavity Plenum : The space between the top of the finished ceiling and the underside of the floor or roof above where used to supply air to the occupied area, or to return or exhaust air from the occupied area
NFPA 90A Standard for the Installation of Air-Conditioning and Ventilating Systems, 2009 ed. http://catalog.nfpa.org/NFPA-90A-Standard-for-the- Installation-of-Air-Conditioning-and-Ventilating-S ystems-Prior-Years-P4155.aspx, (Std: Instll of AC & Vent Sys - Amer) {235 NFC}