Using Floor Joist For Cold Air Return

These sheets of metal make constant noise when we walk on the floor above the return duct.

Using floor joist for cold air return.

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. For example an 8 by 14 inch rectangular duct will fit between 2 by 10 floor joists installed 16 inches apart on center. Panned floor joist are a commonly utilized as return ducts by nailing gypsum board foil insulation osb flooring panels or sheet metal to the joists. Corrective measures are also suggested.

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. Plan your cold air return ducts to fit comfortably in this area. It is impossible to create an airtight seal with this method and together with the negative pressure that is created inside the cavity air leakage occurs through joints of the rim at the end of. Most prefer rigid to flex since flex creates too much resistance and decreases airflow.

Did that cause problems. I have a new house and have a question. Hallways as part of a return air system. All my cold air returns use the space between the joists with sheet metal screwed against the joists.

John here using the joists as a return is common practice if it is inside the envelope conditioned space. Homes with basements often use wall studs and floor joist spaces as return air cavities. If the return lies outside of the envelope insulated rigid or flex ducting is mandatory. The space between the floor joists may be used as part of the return duct system.

The photo below shows some of the joists in the vented crawl space that the basement was completely connected to. This is an appropriate use for space that cannot be used for actual heating and air conditioning. 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.