Using Floor Joist For Cold Air Return

John here using the joists as a return is common practice if it is inside the envelope conditioned space.

Using floor joist for cold air return.

Extensive testing indicates that duct leakage in homes with basements are very high unless the ducts have been pressure tested and sealed under a beyond code program utility state or local that tests for duct. All the duct work for heat uses 5 and 6 inch pipes which is fine and makes no noise. All my cold air returns use the space between the joists with sheet metal screwed against the joists. As a result this panned joist return duct sucked in a lot of air from this basement when the furnace or air conditioner were running.

Still commonly used is the panned floor joist. For example an 8 by 14 inch rectangular duct will fit between 2 by 10 floor joists installed 16 inches apart on center. Plan your cold air return ducts to fit comfortably in this area. The space between the floor joists may be used as part of the return duct system.

If the return lies outside of the envelope insulated rigid or flex ducting is mandatory. This article addresses the issue of unsealed wall stud cavities or panned floor joists used as a return air plenum resulting in risks such as indoor air quality issues due to negatively pressurized cavities drawing air through cracks along with any contaminants and also condensation by drawing in warm humid air. Did that cause problems. Homes with basements often use wall studs and floor joist spaces as return air cavities.

This is an appropriate use for space that cannot be used for actual heating and air conditioning. The photo below shows some of the joists in the vented crawl space that the basement was completely connected to. Using floor joists as return ducts by panning can cause leakage because negative pressure in the cavity will draw air from the outside into the cavity through the construction joints of the rim area at the end of the joist cavity. Most prefer rigid to flex since flex creates too much resistance and decreases airflow.

Hallways as part of a return air system. I have a new house and have a question. These sheets of metal make constant noise when we walk on the floor above the return duct.