In building construction, the term plenum (pronounced PLEH-nuhm, from Latin meaning full) is used to define a space provided for air circulation for heating, ventilation, and air-conditioning (commonly referred to as HVAC). This space is typically located between structural ceiling and a drop-down ceiling. A plenum may also be under a raised floor.

In the event of a fire, most cabling would emit toxic gases which would then circulate through the HVAC system, causing these gasses to be circulated throughout the building. Plenum cabling is often made of Teflon or FEP, which is much more expensive than normal jacketing (e.g. PVC).

Halar sleeving is a much lower cost alternative to Teflon for jacketing bundles of cables. NOTE: putting plenum rated sleeving on your cable assembly does NOT eliminate the need for using plenum rated wire and cable in your system. Please refer to the appropriate NEC (National Electrical Code) and UL guidelines.