Abstract: A collapsible, pop-up plenum for central heating and air conditioning equipment folds substantially flat for shipping and storage. The plenum's planar sidewalls form a rectangular box having an open input end. An endwall opposite the input end is attached by one edge to one sidewall and adapted to swing open and lay against said sidewall, allowing the plenum to fold flat for storage and transportation. A metallic, flanged collar adapted to attach the input end to the equipment cabinet is hinged to collapse with the plenum. Tightness of the plenum's metallized, elastic skin causes the plenum to spring open when released from its wrapping, facilitating assembly and installation.