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PDB Debrief: Return Air Plenums

September 2022


Plenums can be challenging spaces in the construction of a building. Above ceiling spaces are often used to recirculate air back to the fan unit for reconditioning and resupply to the building. The term ‘return air plenum’ is often shortened to simply ‘plenum,” which is defined as an enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of the air distribution system. This can be a very efficient and cost effective air handling approach.


A fire in the World Trade Center in 1975 on the 11th floor quickly spread from the 9th to 14th floors through cable insulation in a plenum space. Along with the fire itself posing a grave danger, the burning material also gave off toxic fumes and smoke, which can exacerbate air quality and visibility during an evacuation event. This event, and others, have spurred changes to the International Building Codes and the International Mechanical Code. The ASTM and the UL have standards for testing materials to be installed in plenums.

This topic touches on both our Quality and Safety pillars. Return air plenums pose a special challenge in construction as many of our buildings are designed this way and requirements for materials installed in plenums are particularly strict. Specific requirements for materials installed in plenums are contained in Section 602 of the IMC, and are listed by many manufacturers as ‘plenum rated’ materials. If the pipe, wiring, insulation, etc, is not listed as plenum rated or does not meet the specific requirements of Section 602, it cannot be installed in a return air plenum.

Using the incorrect materials may cause costly rework, something we wish to avoid. Even more, we want to avoid creating life/safety issues in our projects. This is especially important in areas like plenums that are not regularly observed.


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