An enclosed space used for HVAC airflow, typically at low velocity. A plenum may be constructed of duct materials (e.g., sheet metal) or may utilize interstitial spaces in building construction such as below floors, above ceilings, or in vertical chase-ways. A plenum accomplishes air distribution via static pressure difference between the plenum and surroundings, rather than using velocity and associated diversion/distribution devices, as in ductwork.