Duct Insulation

Any heating or cooling air ducts located in unconditioned areas should be insulated. Insulating your ducts will reduce energy bills and improve comfort. Also, insulating ducts can help to eliminate condensation problems in damp areas.

Types of Insulation

The predominant insulation material for ducts is fiberglass. It can be applied in either a flexible or rigid form and comes in a variety of densities and thicknesses. The flexible blanket insulation is available in rolls for convenience in shipping and ease of application to round or rectangular ducting. Flexible insulation easily conforms to irregular surfaces. Rigid insulation comes in pre-formed boards which are bonded with a thermosetting resin. All duct insulation should have a foil or vinyl facing on the exterior side which serves as a vapor retarder. This prevents moisture from being absorbed into the fiberglass and thus maintains insulation ability. Kraft paper faced insulation should never be used on duct work because of flammability. If any existing insulation has become wet, it should be replaced.

Application

All duct work in attics, basements and crawlspaces should be insulated. The outside air ducts should be insulated up to the outside air damper. The flue pipe should be left bare in the furnace room so that this heat can contribute to heating combustion air.

Insulation can be purchased from a hardware store, or a heating or insulating contractor can install it for you. The most popular thicknesses and their insulation, or R-values, are shown below. Care should be taken during installation to minimize compression.

FIBER INSULATION R-VALUES
Thickness
Flexible
Rigid
1.5"
 4.3"
 5.5"
2.0"
 5.7"
 7.3"
2.5"
 7.1"
 9.1"
3.0"
 8.6"
10.9"
3.5"
10.0"
12.7"
4.0"
11.4"
14.5"