Why is using a stud cavity or joist space as a return air duct a bad idea?
Monday, July 9, 2018
Building cavities that are framed with wood studs and drywall for the purpose of creating a shaft running up to the attic that connects to a return air duct are not as popular as they were years ago, and for good reason: they leak and are difficult to maintain. Also, condensation can on the uninsulated surfaces can be a problem.
The current building code allows them only for return air (not supply air) and they cannot convey air from one floor to another. The photo above was taken looking up into a stud-framed cavity to a duct connection at the attic level of a 1980s house. Yes, the cobwebs are real and so are the seams and gaps in the drywall in the floor of this cavity shown below, with dust/dirt accumulation from the suction of unfiltered air from adjacent spaces around the unsealed base plates.
Because the air filter is usually placed just behind the return air register at the bottom of the cavity, everything that gets pulled through the tiny openings in the shaft ends up stuck to the evaporator coils in the air handler. This configuration is approved by the building code, so we don’t recommend repair, but it a less than ideal component of an HVAC system.
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