Above Ceiling Return Air Plenum

Dropped ceilings or suspended ceilings are often installed so that the space between them and the floor above or the roof deck can be used as a return air plenum.

Above ceiling return air plenum.

A commercial building with a plenum airspace. Vertical section through a commercial building without a plenum airspace. This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. In thermal applications the area above the insulation should not act as a return air plenum.

Typically spaces between the structural floor or a dropped ceiling are considered plenum. 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. When installing a return air plenum system it is absolutely essential that the system is constructed airtight without any leaks above the ceiling space. 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.

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. This is most common in commercial buildings schools hospitals or libraries. Return air is then directed into the grille and travels to each crac unit. Thermal insulation in return plenum areas would best be added at the roof and side walls.