What is a Plenum Space?

A plenum space is simply any enclosed space within a building that is used for airflow. Because they are typically rich in oxygen, they pose an potential risk to a building in the event of a fire. Examples of plenum spaces would be the air ducts above the ceiling and other ventilation shafts. Ever notice how even in wooden buildings the ventilation shafts are metal? Just think what would happen if you had wooden ventilation shafts and a fire started in an obscure room away from open sight. Within minutes that fire would spread through the ventilation system and threaten the entire building!

 

Because of the dangers of enclosed airflow, these ventilation spaces are designated as Plenum and treated differently than other spaces inside a building, sort of like zoning regulations within a city. There are certain rules and restrictions in place that regulate what sorts of material and activity are permitted within a plenum-rated space.

 

The etymology of the word is based on a 17th century term that described the opposite of a vacuum. That is, free-flowing air indicated a lack of vacuum. Thus plenum-based systems are the opposite of vacuum-based systems.