Homeowners want the best performance from their ducts and sheet metal ducts still provide the most durable, efficient and clean air distribution system. In addition to offering superior air flow, the easiest way to install sheet metal duct is in a straight line, so installers tend to limit the number of turns and to plan them such that each run can go as far as possible before changing directions.
The result is a galvanized metal duct system with the fewest number of turns which limits friction losses and increases efficiency. Zinc in the galvanized coating of sheet metal duct actually kills mold and gives excellent protection against all types of organic growth. This can be very beneficial as we have discovered in recent years. Duct systems do need cleaning occasionally and the hard, smooth surface of sheet metal can be cleaned and maintained far better than nonmetallic duct materials.
Sheet metal duct is hard and smooth, it conducts air very efficiently. It also conducts sound efficiently. Flex duct has a rough surface, so it isn't the most efficient way to move air, but it does have superior sound attenuating qualities.