A duct is a round or rectangular tube, generally constructed of sheet metal, fiberglass board, or a flexible plastic-and-wire composite, located within a wall, floor, and ceiling that distributes heated or cooled air in buildings.


Think of your ductwork as huge hoses, delivering hot air or conditioned air instead of water into your house. Mostly out of sight, ducts can leak for years without you knowing it. They can become torn or crushed and flattened. Duct tape – the worse thing with which to seal – dries as it ages and falls off, allowing junctions and splices to open, spilling heated or conditioned air into your attic or under the house.


In new home construction or in retrofits, proper duct system design is critical. In recent years, energy-saving designs have sought to include ducts and heating systems in the conditioned space. See duct design and installation for more about this.


Many existing duct systems lose a lot of energy from leakage and poor insulation. Leaky ducts waste heated and cooled air even before it arrives inside your carefully weatherstripped building envelope. Existing ducts may also be blocked or may require simple upgrades. Refer to the various entries below for more information.