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.

Above ceiling return air plenum.

A commercial building with a plenum airspace. This is most common in commercial buildings schools hospitals or libraries. The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum. Vertical section through a commercial building without a plenum airspace.

Return air is then directed into the grille and travels to each crac unit. When installing a return air plenum system it is absolutely essential that the system is constructed airtight without any leaks above the ceiling space. With return air plenums there is less control over where the air actually travels. 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. 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. 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.