What Types of Ducting Should I Use in the Attic?

Use Flexduct

• Use flexible duct -- flexduct -- wherever possible in an attic. It is flexible, bending around roof trusses and rafters, vents and other obstructions. It is round,and made of wires covered with fiberglass insulation and an air-tight outer layer of metallic material. Flexduct is easy to cut to length and to install. Lay flexduct over joists in an attic and fasten it in a few places with metal straps that go over the duct and nail to joists on each side to prevent the duct from blowing in any wind that gets in.

Ductboard

• Build ducts of foamboard or ductboard for areas where flexduct does not fit. There may be spots in an attic where a 12-inch round duct cannot go. For those spots, make square or rectangular ducts of ductwork. Ductwork easily cuts with a knife and can be bent into shapes to fit between truss elements or beside vent pipes. It can be sealed with heat-resistant tape and can be connected to flexduct with metal connectors and tape.

Metal Ducts

• Make ducts of sheet metal for any areas where neither flexduct or ductboard work. An example might be around a chimney; flexduct and ductbord should not be installed close to chimneys or other areas with high heat, but metal works in those locations. Sheet metal needs to be formed with special equipment and may require professional assistance.

Insulate Ducts

• Install all attic ducts over joists. Insulate thoroughly between the joists before installing any attic ducts. Add extra fiberglass batt insulation around attic ducts to minimize heating and cooling loss. Keep attic ducts away from walls and in the center of the attic as much as possible.