Any heating or cooling air ducts located in unconditioned areas should be insulated. Insulating your ducts can reduce energy bills and improve comfort. Also, insulating ducts can help to eliminate condensation problems on metal ducts in humid climates.
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 (in some areas, ducts are constructed of a rigid insulation material, minimizing the need for additional insulation) . All duct insulation should have a foil or vinyl facing on the exterior side to prevent moisture from being absorbed into the fiberglass. 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 (a minimum of R-5 is recommended in most situations). Check with your local utility or a qualified heating and air conditioning contractor for the recommended R-value for your application.
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Installation
Before installing duct insulation, you should seal any observable leaks or loose joints with duct mastic or foil tape. Then install the insulation, making sure that that the vinyl or foil backing faces outward. Avoid compressing insulation, as compression reduces the effective R-value. Once the insulation is installed, seal all seams with duct mastic or foil tape.
If possible, avoid using the traditional cloth "duct tape" which tends to come loose over time. Either duct mastic or foil tape will form a tighter seal and last longer, particularly in humid areas.
