Stud cavities and spaces between floor joists are often used as return air paths. Unless these paths are tightly sealed, return air may be drawn into central equipment from unconditioned spaces such as hot or cold attics, basements, and crawl spaces, and supplied to occupied spaces. Such unwanted air can easily enter a non-ducted return system through numerous construction openings such as wiring conduits and chases.

Fig. 33 shows ways in which leakage can be minimized when stud and joist cavities are used as return ducts.

The builder and the HVAC contractor should work together in the early stages of construction to ensure opportunities to provide for return air systems when it can still be done right - before interior walls, ceilings, and other construction block access to potential locations for return air paths.