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 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. 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. Vertical section through a commercial building without a plenum airspace.

This is most common in commercial buildings schools hospitals or libraries. 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. In thermal applications the area above the insulation should not act as a return air plenum. With return air plenums there is less control over where the air actually travels.

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. The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum. Typically spaces between the structural floor or a dropped ceiling are considered plenum.