Using Floor Joist For Cold Air Return

Still commonly used is the panned floor joist.

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. This is an appropriate use for space that cannot be used for actual heating and air conditioning. If the return lies outside of the envelope insulated rigid or flex ducting is mandatory. The photo below shows some of the joists in the vented crawl space that the basement was completely connected to.

All my cold air returns use the space between the joists with sheet metal screwed against the joists. Most prefer rigid to flex since flex creates too much resistance and decreases airflow. Did that cause problems. All the duct work for heat uses 5 and 6 inch pipes which is fine and makes no noise.

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. These sheets of metal make constant noise when we walk on the floor above the return duct. Hallways as part of a return air system. 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.

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. Homes with basements often use wall studs and floor joist spaces as return air cavities. Corrective measures are also suggested. I have a new house and have a question.

John here using the joists as a return is common practice if it is inside the envelope conditioned space. 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. For example an 8 by 14 inch rectangular duct will fit between 2 by 10 floor joists installed 16 inches apart on center.