Flex duct
Also called: flexible duct, flexible ductwork, insulated flex, R-8 flex duct
Flexible air duct: a wire helix inside a plastic inner liner, wrapped in fiberglass insulation (R-6 or R-8) and a foil or plastic outer jacket. Sold in 25 ft boxes at $45–$65; the standard branch duct in attics and crawl spaces, and the one a homeowner can install.
Flex duct is cheap, quiet and fast to install: it bends around framing without fittings and connects to a collar with two straps and a brush of mastic. It lasts 10–20 years — the inner liner goes brittle and the jacket tears, and rodents open it — against 30–50 for sheet metal.
Its weakness is that its capacity depends on how it’s hung. The inner liner is ribbed, so friction is already higher than smooth metal; compress it, let it sag, snake the excess through the attic, or bend it tightly and the airflow drops — a run with 30% compression can lose a third or more of its rated air. The rules that matter: pull each run tight and cut off the excess, support every 4 ft with straps at least 1.5 in wide, no more than 1/2 in of sag per foot, bends no tighter than one duct diameter, and never let it rest on a joist edge or pipe.
Sizes are named by inner diameter: 6 in (about 100 CFM) for a bedroom, 7 in (about 150) for a larger room, 8 in (about 200) for a living area, 10–16 in for returns and short trunks. Changing a size without a Manual D check usually starves another room. Code in most places requires R-8 insulation on ducts in unconditioned attics and crawl spaces.