Above Ceiling Return Air Plenum

Vertical section through a commercial building without a plenum airspace.

Above ceiling return air plenum.

This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. Normally the space above the ceiling and the deck above is used to return air from the conditioned spaces below the ceiling to the air handling equipment. Ducted return air plenum this method requires the installation of ductwork that runs from each computer room air conditioning crac unit up into the space above the suspended ceiling and to each return air grille. With return air plenums there is less control over where the air actually travels.

Typically spaces between the structural floor or a dropped ceiling are considered plenum. When installing a return air plenum system it is absolutely essential that the system is constructed airtight without any leaks above the ceiling space. A commercial building with a plenum airspace. When both the supply and return ducts are constructed in this manner it is possible to insulate the ducts and the dropped ceiling so that the upper airspace is not heated or cooled increasing energy efficiency.

Thermal insulation in return plenum areas would best be added at the roof and side walls. In thermal applications the area above the insulation should not act as a return air plenum. Return air is then directed into the grille and travels to each crac unit. The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum.