AskCodeMan

AHU closet, Flex duct penetrate through drywall ceiling on the second floor and did not have proper support strap. Is flex duct approved to penetrate the drywall ceiling?

Hi Richard,

Flexible non-metallic duct is allowed to pass through a wall, floor or ceiling ... unless that wall, floor or ceiling is a fire-rated assembly.

However, the problem becomes one of properly sealing around the flexible duct without constricting the outer covering and insulation such that the R-value of the insulation is reduced (any compression of insulation reduces the R-value of that insulation, so that part becomes an easy question to answer). The penetration space around the duct to the drywall needs to be sealed to prevent air infiltration and exfiltration, and that sealed aspect is difficult to maintain with flexible duct which moves, sags, and settles. It is most pronounced when penetrating horizontally through a wall when the duct settles and sags, leaving a large open gap between the duct and the drywall.

Thus, yes, penetrating a wall, floor, or ceiling with flexible duct is allowed, it is just not good basic construction practice. The best way would be to penetrate through with duct board, install a collar around the outside to attach it to the ceiling, then properly attach the flexible duct to that duct board, but ... code only addresses minimum, not good, better, or best construction practices. The better HVAC contractors will make the transition to duct board through the ceiling, floor, or wall, then back to flexible duct, or run the entire main trunk lines in duct board, but, alas, that is one thing which makes them "better", just "not required".