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 humid areas.

Types of Insulation

The most common insulation material for ducts is fiberglass. It is available in either a flexible or rigid form and comes in a variety of densities and thicknesses. The flexible blanket-type insulation is sold in rolls and is easy to apply to either round or rectangular ducts. Flexible insulation easily conforms to irregular surfaces. Rigid insulation comes in pre-formed boards bonded with a thermosetting resin, and works best on rectangular ducts. All duct insulation should have a foil or vinyl facing on the exterior side to prevent moisture from being absorbed into the fiberglass and maintain insulation effectiveness. Kraft paper-faced insulation should never be used on ducts because of its flammability and relatively poor moisture resistance. If any existing insulation has become wet, it should be replaced.

Thickness And R-Value

Insulation can be purchased from a hardware or building supply store, or a heating or insulating contractor can install it for you. The most popular thicknesses and their insulation or R-values ("R" stands for "resistance," and is the standard measurement index for all insulation) are shown below. The recommended R-value for duct insulation depends mainly on the temperature difference between the air in the ducts and the surrounding air. Check with your local utility or a qualified heating and air conditioning contractor for the recommended R-value.

 

Fiberglass Insulation R-Values
Thickness	Flexible	Rigid
1.5"	4.3	5.5
2"	5.7	7.3
2.5"	7.1	9.1
3"	8.6	10.9
3.5"	10.0	12.7
4"	11.4	14.5

Installation

You should seal all observable leaks and loose joints with duct mastic or foil tape prior to installing the insulation. The ducts can then be wrapped with the insulation, ensuring that the vinyl or foil backing faces outward. Care should be taken during installation to minimize compression. Be sure to seal all seams with duct mastic or foil tape to prevent leakage of heated or cooled air into unconditioned areas of the home.

If possible, avoid using the traditional cloth "duct tape." Either duct mastic or foil tape will form a tighter seal and last longer, particularly in humid areas.