A flex duct is a type of duct that consists of two grey PVC-layers and one layer of reinforced fiberglass. Flex ducts are convenient for attaching supply air outlets to the rigid duct work.
Flex ducts have a wide use such as in heating, ventilation and air conditioning. They have a variety of configurations. In the United States, the insulation is usually glass wool, but other markets, such as Australia, use both polyester fiber and glass wool for thermal insulation. Unlike a typical duct that does not tolerate any negative pressure, a flex duct can tolerate moderate negative pressures. Flex duct is available in diameters ranging from 4 inches to 18 inches.