Leaky supply ducts in attics and crawlspaces dump conditioned air outdoors, and leaky returns pull in hot, humid or dusty air, depressurizing the house. Leakage is measured with a duct pressurization fan as cfm at 25 pascals, and energy codes cap it relative to floor area. Sealing joints with mastic or approved tape is one of the cheapest efficiency and comfort fixes there is.
Duct leakage test came in at 280 cfm25, half of it from the return platform, mud every seam in that closet.
Also heard as
- duct leaks
Related terms
Mud
Duct mastic, the thick paste brushed over duct joints and seams to seal them airtight.
Static pressure
The pressure air exerts outward on duct walls, the resistance a fan works against, measured in inches of water column.
Plenum
A box on the supply or return side of an air handler that feeds or collects several ducts, or a ceiling cavity used as a return path.
Infiltration
Outdoor air that sneaks into a building through gaps in its shell, pushed by wind, stack effect and fans rather than by design.