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Distribution System & Duct Leakage

Having leaking ductwork not only lowers the efficiency of the HVAC system, it can also negatively impact your indoor air quality and overall home comfort.

- Efficiency: According to the US Department of Energy, “You can lose up to 60% of your heated air before it reaches the register if your ducts aren’t insulted and they travel through unheated spaces, such as the attic or crawlspace.” In the homes we test, we have found that most duct systems leak an average of 30-40%. That is the equivalent of throwing away $0.40 for each dollar you spend on heating and cooling!

- Air Quality: Ductwork is usually located inthe attic or crawlspace, which can cause harmful indoor air quality. During the winter months, the crawlspace usually gets damp, which combined with the dark environment, encourages mold growth. Every time the heater comes on, the leaking ducts can pull this damp, moldy air into the home!

- Comfort: A very common concern of homeowners is that some rooms are too hot or too cold, and others don’t receive enough air flow. Older duct systems don’t have dampers ,which help balance air flows by room and direct heat where you want it.Fixing leaks and installing dampers can alleviate most of these concerns.

Due to the catastrophic duct leakage throughout the duct system, the ducts could not be pressurized to determine leakage. Large rips and gashes are allowing conditioned air to escape the home. All ducts should be replaced for maximum efficiency.

Ductwork Questions & Answers

• Sealing • Insulating • Benefits • How-to

We spend hundreds of dollars on energy efficient fatures, but recent studies on Southern homes show that we often overlook plugging the biggest energy waste-leaks in the ductwork for forced-air heating and cooling systems.

Q. Why is sealing the ductwork important?

A. Leaky ductwork often accounts for 10 to 30 percent of total heating and cooling costs. For an average home, leaky ducts can waste hundreds of dollars each year.

While the increase in energy costs is significant, protecting health and safety is the most important reason to seal ducts. Ducts are usually located in the attic, crawl space or basement. If the return ducts leak, they draw air from these areas directly into the home. This air can be contaminated with dust, mold, excess humidity, and potential toxins such as pesticides for termite treatment, combustion gases, and radon. When supply ducts leak, they can create a lower pressure inside the house which draws in contaminants, too.

Q. How do duct leaks affect heating and cooling equipment?

A. If the duct leakage is 20% of the total air flow, the efficiency of the cooling system can drop by 50%. Heating efficiency is similarly affected. Duct leakage also lowers the heating and cooling capacity, and can lessen equipment life. Many comfort complaints in homes are due to poor ductwork.

Air Handler

Q. How do I know if a system has duct leaks?

A. Start with a visual inspection of the system. Energy codes require that all joints in the ductwork be sealed. However, many leaks are not readily visible. To determine if you have serious leakage requires a pressure test of the ducts with a special fan.

The test involves temporarily taping over the registers, then blowing air into the ducts with the fan to determine the amount of leakage as well as the location of the leaks. The test takes about an hour. Some contractors conduct the test for a nominal fee or include it as part of a bid to seal leaky ductwork.

Q. Why is duct leakage so common?

A. Most connections are simply not sealed. There is also a problem with poor quality materials being used to seal ducts. Duct tapes do not provide a permanent seal. Their adhesive dries out. Experts recommend duct sealing mastic. Mastic is a thick paste which can be used on all duct materials and provides a permanent seal. Mastic comes in tubs and tubes and costs around $10 per gallon. For new construction, the cost of material and labor to seal all the joints with mastic should be about the same as for a quality job sealed with tape.

Supply and Return Plenums


Flex Duct


Q. If the ducts are insulated, do they need to be sealed?

A. Yes. Insulation does not stop air leaks. Look for dirt streaks in duct insulation-it's a sign that air has been leaking from the ducts.

Q. Are certain types of ducts more airtight?

A. Studies done across the Southeast show that all types of ductwork can have problems with air leakage. Mastic works to seal metal, flexible, and fibrous ductwork. Try to avoid using framing (such as panned returns) for ductwork as these create hard to seal leaks.

Q. Where is the best location for ductwork?

A. The problems due to leaky ducts can be reduced or elimi­ nated by bringing the ducts inside the conditioned space. If that is not possible, the crawlspace or basement is usually preferred over the attic due to the summertime heat buildup in the attic. Exterior walls are bad locations for ducts because they displace wall insulation and can cause condensation problems.

Q. Where are the most important areas to seal?

A. Seal in this order:

High Priority Leaks

  • Disconnected components
  • Connections between air handling unit and the plenums
  • All seams in the air handling unit, plenums and rectangular ductwork, especially in hard to reach places
  • Return takeoffs,boots,and other connections,
  • especially site-built items

Moderate Priority Leaks

  • Joints between sections of branch ductwork

Low Priority Leaks

  • Longitudinal seams in round metal ductwork

Air Barrier FAQs

Q. Why install an air barrier?

A. Chances are good that right now, you are already installing air barriers: they are required not only by Energy Star version 3.0, but also by California’s T24 code and all recent versions of the IECC code.

Q. What does an air barrier do?

A. Air barriers seal your thermal envelope to minimize both unwanted air movement and moisture through or into the assembly. Without a proper barrier, even a solid-looking structure can fall prey to this kind of air and moisture movement—it’s not just wind, but also pressure differences that act to force unwanted air through the building’s assemblies. These pressure differences are the most pronounced between the interior and the exterior of the building envelope, and have a dramatic effect on the dwelling’s heating and cooling loss.

Q. What material can be used as an air barrier?

A. The real question is: what methods of construction qualify as a properly installed air barrier? An air barrier is a system of materials designed to control airflow between a conditioned space and an unconditioned space. These products include, but are not limited to, rigid sheathing materials such as gypsum board, plywood, OSB, Thermo-Ply®, or rigid polystyrene sheathing. They all must be:

  • Impermeable to airflow
  • Continuous around the entire building enclosure
  • Able to withstand the forces and elements that may act on them during and after construction, and
  • Durable over the expected lifetime of the building. Depending on your location and the programs and codes you are working with, there may be additional requirements. If you are unsure, let us know—we are here to help!
Download the free PG&E Duct Testing brochure

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Sierra Pacific Home & Comfort, Inc.
2550 Mercantile Drive Suite D
Rancho Cordova, CA 95742
(800) 551-3040

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