Above Ceiling Return Air Plenum

Vertical section through a commercial building without a plenum airspace.

Above ceiling return air plenum.

Return air is then directed into the grille and travels to each crac unit. This would render the insulation thermally ineffective as the air space above the ceiling would be at the same temperature as the room below. 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.

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. Typically spaces between the structural floor or a dropped ceiling are considered plenum. In thermal applications the area above the insulation should not act as a return air plenum.

Thermal insulation in return plenum areas would best be added at the roof and side walls. 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. A plenum space is the part of a building that enables air circulation by providing pathways for heated conditioned and return airflows at a higher pressure than normal. With return air plenums there is less control over where the air actually travels.