Flex Duct vs. Metal Duct: Where Each Belongs

Metal duct wins on airflow: smooth interior, low friction, holds its shape forever. Flex wins on speed and cost for short connections. Neither is 'wrong' — the failures come from using flex where metal belonged.

Flex is at its best on short, straight, fully stretched runs from trunk to boot. Compressed inner core, long snaking paths, sags between supports, and tight bends can double or triple its friction — that's where 'flex is junk' stories come from.

Returns deserve special respect: they run at higher effective friction and every restriction shows up as blower strain. If a return must be flex, oversize the diameter and keep it short; hard pipe returns are the difference customers can hear at night.

The WNC wrinkle: in vented crawlspaces, insulation grade matters as much as material — R8 flex or well-wrapped metal, because summer condensation doesn't care what the duct is made of, only how cold its skin gets.