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Duct Insulation

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.

Fiberglass Insulation: Typical R-Values


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.

Duct Sealing

Sealing leaks in your heating and air conditioning duct system can be one of the most effective energy improvements to your home. Studies have shown that leaky duct systems can increase heating and cooling costs by as much as 20% or more. Duct sealing and repair can provide an opportunity for significant energy and cost savings, while also improving comfort.

During the winter months, duct systems supply heated air throughout the home through supply ducts and return cooler air through return ducts for filtering and reheating. In the summer, duct systems supply cooled air throughout the home and return warmer air for filtering, moisture removal, and recooling.

Leaks in your ducts allow air to escape from or enter your duct system, increasing your energy costs. Leaks in supply air ducts will waste energy by allowing heated or cooled air to escape into unconditioned areas like crawl spaces or attics. Leaks in the return air ducts have the opposite effect. These leaks allow unconditioned air from those same spaces to enter the duct system, mixing with the heated or cooled air in the ducts.
Duct systems are often hidden from view, which makes finding leaks difficult. Fortunately there are ways to test and repair ducts to improve the energy efficiency of your home and reduce your heating and cooling costs.

What Is Duct Testing and Repair?

Duct testing and repair is the process of identifying and sealing leaks in the home's duct system. The standard procedure starts with a visual inspection of the duct system. This is followed by a duct pressurization test, which should be conducted by a qualified heating and air conditioning contractor or testing organization. This test, often called a "duct blaster" or "blower door" test, determines the amount of air leakage in your duct system and where it is occurring. Common leakage sites are around air registers and grilles, duct connections, panned floor joists, and the connection to the heating and air conditioning unit itself.

Repairs to ducts are made using a special adhesive called "mastic". Mastic is applied over a mesh tape to form a seal over gaps or leaks in the ducts. This seal will last for many years, unlike old-fashioned cloth duct tape. Other acceptable methods include: silicone caulk, duct sealer, and special foil tape. Duct sealer is available in cans and caulk tubes. Check the label, and purchase a sealant that is nontoxic. New improved foil tapes have better adhesive and cold weather characteristics than before. There is also a type of "mastic tape" available, which incorporates a coating of mastic bonded to a foil tape. Always follow the manufacturer's recommendations and make sure ducts are dirt-free before applying any type of foil tape.

Once repairs are made, a second "duct blaster" or "blower door" test can be conducted to confirm the reduced air leakage.

Caution: Duct repairs attempted by untrained personnel may have an adverse effect on the home's occupants. Anyone repairing duct leaks and duct systems should be experienced and familiar with how duct modifications can affect living conditions and air quality.


  • Lower heating and cooling bills
  • Improved humidity control
  • Increased comfort
  • Improved indoor air quality
  • Improved equipment efficiency

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