International Residential Code

IRC M1502.4.6

IRC limits dryer exhaust to 35 feet maximum developed length (including equivalent length for fittings). At 40 feet equivalent, this exceeds the maximum. Options: reroute for a shorter path, eliminate one elbow, or install a dryer exhaust booster fan listed for the application. Excessive length causes lint buildup — a leading cause of house fires.

IRC M1507.2

Exhaust ducts must terminate outdoors, never in an attic, crawl space, or soffit. Moisture buildup in attics causes mold, wood rot, and structural damage.

IRC M1601.1.1

Building cavities (joist bays, stud spaces) can be used as return air pathways in residential construction when properly sealed with sheet metal panning. This is a common and code-compliant practice in residential systems. However, supply ducts cannot use building cavities — only return air.

IRC M1601.1.2

Sheet metal duct is allowed in garages when properly sealed. The key requirement is that ductwork in garages must not have openings (registers/grilles) in the garage space to prevent carbon monoxide from vehicle exhaust from entering the duct system. Sealed sheet metal running through the garage to serve other spaces is acceptable.