Duct Leakage, Testing and Sealing

If your home uses forced air for heating and/or air conditioning (as most do), you could be wasting a great deal of energy through leaks in your duct system. Studies by the U.S. Department of Energy, Lawrence Berkeley Laboratories and others have shown that up to 25% of the average home's heating and cooling energy is wasted through inefficient duct systems. Finding and sealing leaks in your duct system can result in significant energy and cost savings, while also improving indoor comfort.

During the winter months, heated air is distributed throughout the home through supply ducts, and room air flows back to the heating system through return ducts for filtering and re-heating. In the summer, duct systems supply cooled air throughout the home and return warmer air for filtering, moisture removal, and re-cooling.

Air can leak into and out of ducts at all the connections in the system - between the furnace or AC unit and the ducts, at branches in the duct system or any joints between sections of duct, and at the connections to the registers or vents. Leaks on the supply side of the system cause heated or cooled air to escape to attics or other unconditioned spaces, reducing the amount available to heat or cool the home. On the return side, leaks draw unconditioned air into the system, which must then be either heated or cooled and dehumidified. These leaks can also cause pressure imbalances within the home, increasing the amount of infiltration or air leaking into and out of the home. Even with the system turned off, a leaky duct system increases the ventilation rate in the home and the need for heating or cooling.

Duct systems are often hidden from view, which can make finding leaks difficult. Fortunately there are tests that can be used to determine the extent of the problem and the location of any leaks. The leaks can then be properly sealed to improve the energy efficiency of your home and reduce your heating and cooling costs.

Duct Testing

A duct system test uses one of several methods to force air through the duct system under pressure and measure the air flow at different locations within the system. By comparing the pressures and flow rates, the test is able to determine the total amount of leakage and the location of any leaks in the system.

One testing method uses a device called a Duct Blaster®, which pressurizes and forces air through the duct system with all supply and return vents sealed. The air flow is then measured at different points throughout the system, and any reductions in pressure or air flow indicate the presence of a leak. A qualified contractor can typically test the system and seal any leaks in a single visit.

A second method uses a device called a blower door to pressurize both the house and the duct system and measure pressure differences within the system. The blower door can also be used to test the home for infiltration or air leakage (see "Air Sealing Measures"). There are also several other testing methods currently being evaluated.

Sealing Leaks

Once the leaks have been located, they can be sealed with duct mastic (a thick paste that forms a tight permanent seal), foil tape (there is a version with a mastic backing that works well) or aerosol sealants. Traditional cloth duct tape is not recommended, as it tends to come loose over time. After the repairs are completed, a second duct test can be performed to confirm the reduced air leakage.


If a significant portion of your heated or cooled air is lost through leaks in the duct system, your heating and cooling system must run longer to deliver the desired degree of comfort, increasing your energy costs. A four Ton air conditioner connected to a duct system that loses 25% of the air via leaks is only delivering the equivalent of three Tons (but you're still paying to run a four Ton unit). Sealing the leaks will not only reduce your heating and cooling expenses, but may enable you to down-size the unit when the time comes for replacement.

A properly sealed duct system can also improve comfort by shortening the time it takes for the house to cool down after the system has been off or the thermostat setting raised.

Eliminating duct leaks can also improve indoor air quality by preventing dust, pesticides, moisture and other pollutants from being drawn into the duct system from attics or crawlspaces.

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