Above Ceiling Return Air 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.

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. With return air plenums there is less control over where the air actually travels. Vertical section through a commercial building without a plenum airspace. Return air is then directed into the grille and travels to each crac unit.

A commercial building with a plenum airspace. This is most common in commercial buildings schools hospitals or libraries. Typically spaces between the structural floor or a dropped ceiling are considered 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.

Thermal insulation in return plenum areas would best be added at the roof and side walls. 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. In thermal applications the area above the insulation should not act as a return air plenum.