What is a plenum?
A plenum is an enclosed void in a building that serves as an air chamber: air is collected in it or distributed from it. In buildings that is usually the space above a suspended ceiling or beneath a raised access floor, but the distribution box behind a grille carries the same name.
The word comes from the Latin plenus, “full” — the space sits at a slight positive or negative pressure relative to the room, which spreads the air evenly across the connected grilles.
Three forms
- Ceiling plenum. The space between the suspended ceiling and the structural soffit above is used as the return air path in its own right. There are no separate extract ducts; air passes through ceiling grilles into the void and is drawn off centrally. This saves ductwork and floor-to-floor height.
- Floor plenum. With a raised access floor (data centres, offices with flexible workstations) cool air is blown into the floor void and supplied through floor grilles wherever it is needed.
- Equipment plenum. A box directly behind a grille, an air distribution box, or the intake chamber of an air handling unit. It settles the airflow and evens out the velocity across the grille face.
What to watch
- Fire safety. A plenum connects rooms to each other. Where the void passes over a fire- or smoke-resisting separation, that separation must be continued within the plenum; otherwise the compartmentation is worthless. This is one of the most common workmanship failures with suspended ceilings.
- Materials in the void. If the space is used as an air path, that places requirements on the smoke production of everything inside it: cables, insulation, adhesives.
- Airtightness. A ceiling plenum only works if the void is reasonably sealed. Unintended leakage into the riser or at the facade destroys the distribution.
- Fouling. A plenum used as a duct cannot be cleaned internally the way a duct can. Dust accumulates on services and on the top of the ceiling tiles.
- Sound. A continuous plenum carries noise from room to room — flanking transmission. For speech privacy you need a partition carried up to the soffit, or at minimum a heavy absorbent barrier in the void.
Related terms
- Mechanical ventilation
- Balanced ventilation
- Ceiling hanger
- Airtightness
- Suspended ceiling
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.