Plenum

In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning full) is a separate space provided for air circulation in the form of heating, ventilation, and air-conditioning (sometimes referred to as HVAC) and typically provided in the space between the structural ceiling and a drop-down ceiling. A plenum may also be under a raised floor. In buildings with computer and/or audio/video installations, the plenum space is often used to house connecting cables, as opposed to special conduit, which is designed to isolate cables from environmental factors to some degree. Because the jacket and insulators used ordinary cable can introduce a toxic hazard in the event of fire, building codes normally call for special plenum cabling in plenum areas. Plenum cable is more resistant to flames and when it does burn produces less smoke than standard cable. And, of course, it costs more.