Article Summary

Your air distribution system or ductwork carries conditioned air between the central air handler and rooms throughout the home. (The terms air distribution system, duct system and ductwork will be used interchangeably throughout this article) The duct system is as important as your heating and cooling equipment, yet it is often the most neglected part of the system. Airtight ductwork is one of the keys to a quality heating and cooling system. Conditioned air should not leak into attics and crawl spaces, nor should return ducts draw outdoor air in from these areas. Duct leaks, as this article will explain, will have an impact on comfort, energy efficiency, indoor air quality and in some extreme situations – result in serious health and safety concerns.

Compared to other energy conservation improvements, duct sealing is relatively inexpensive and can yield significant energy savings. It is critical that the air distribution system be properly sealed and pressure tested to ensure it meets national airtightness standards. We work in low income weatherization programs in Phoenix where every system we seal is pressure tested to make sure it meets national standards. Your typical duct system that is accessible can be sealed and tested for $800 to $1,400. APS and SRP are offering rebates as an incentive to get this work done. APS estimates a homeowner can save up to $200 a year by having their duct system sealed. Advantage Home Performance has the specialized equipment, HVAC license and in-house expertise to design, seal, test and modify air distribution systems.

An in-depth discussion of duct leakage, combustion safety, pressure imbalances and indoor air quality, etc. is well beyond the scope of this article. Our goal is to provide a very informative overview of duct leakage and repair for homeowners. For our readers who want more in-depth information, at the end of this article we have provided several informative links including an article we wrote on duct leakage for the Journal of Light Construction.

Seventy percent of homes have significant duct leakage

An EPA-sponsored report titled, National Energy Savings Potential from Addressing Residential HVAC documented that 70% of homes have significant duct leakage with a potential savings of 17%. The building code did not require duct work to be sealed and pressure tested until 2009. The two photographs below are of a major duct leak. The flex duct literally slipped off a plenum creating a major disconnect right next to the air handler. These photographs highlight just how sloppy the HVAC industry had gotten in regard to installation.

Thirty-five years of neglect, ignorance and shoddy workmanship

Poorly installed and leaky air distribution systems were the rule from the early seventies until recently. Air distribution systems were treated as an afterthought. Speed of installation trumped quality and ignorance compounded the problems. In most cases the heating and air conditioning equipment was so grossly oversized that it kept a home heated or cooled regardless of the duct leaks.

We routinely find duct systems that are so leaky we cannot test them. They are wide open to the attics or crawlspaces where they are installed. Framed return cavities are huge offenders (see photo below on right). It has only been in the past 10 years that builders, building officials and utilities started to educate and require air tight ductwork.

Duct leakage & indoor air quality

Over the past two decades we have seen a large number of indoor air quality, health and safety problems created by duct leakage. Particulates, dust, allergens, combustion byproducts, pesticides, radon, humidity, etc. are all transported by airflow. You can see a dead rodent in the photograph below. Duct leakage and pressure imbalances due to duct leakage and door closure will play a major role in the transport of these indoor pollutants.

Return duct leaks are responsible for many indoor air quality problems. Return ducts draw air back to the unit to be reheated or cooled. The return duct is under a negative pressure because the blower is sucking air back to the unit. Return ducts are often located in attics, garages and crawl spaces. If a return duct leaks it will suck air back from the space it is located in. As you observed in the earlier example, the leaky return ductwork was sucking dusty air down an interior wall and distributing the dust throughout the house.


Combustion safety is a major concern. One of our goals is to make sure combustion byproducts, such as sulfur oxides, nitrogen oxides, moisture and possibly carbon monoxide, all flow up the chimney and are vented to the outdoors. Pressure imbalances created by duct leakage in mechanical closets can reverse the flow of chimneys, thus drawing combustion by products into the home. The above photograph on the left hand side shows an auditor with a smoke pencil watching a water heater backdraft due to return duct leakage in a mechanical closet. To minimize combustion safety risks, combustion safety testing should be conducted after duct sealing is performed.

Commissioning duct systems

There is only one way to make sure that the duct work has been thoroughly sealed and that is to test it. A simple pressure test can measure the airtightness of the air distribution system. You need to keep in mind that no duct system will be completely airtight. The duct blaster provides a direct measurement of total duct leakage. We can compare the duct leakage in your system against national standards. We can also use a blower door and pressure pan to measure duct leakage.


There is a big difference between sealing ductwork and sealing it correctly. Just because you see mastic applied to a duct system, mastic in of itself does not ensure that the system is sealed. In the photograph below you can see how a technician globed on the mastic which is white and then the technician or a fellow crew member must have tugged on it and left a gaping hole in this return. It is obvious in this photograph that the technician who performed is simply going through the motions. He was probably told to perform duct sealing, but most likely never participated in any meaningful training or testing. We see many systems where the duct sealing was only partially completed.

Duct airtightness standards

On our new construction projects we rely on a duct blaster to test our duct systems. We participate in the Energy Star Program as both a HERS Rater and an Energy Star Certified HVAC contractor. The duct sealing standard we test to in new construction is [{square footage of house ÷ 100} x 4 = not to exceed duct leakage]. The goal is not to be at 0 cubic feet of air leakage because the air handlers have operable doors that leak a little. A two thousand square foot home would be permitted to have 80 cfm of leakage. For a trained crew this is a very easy target to hit. The 2009 International Energy Conservation Code (IECC) has a much more lenient duct leakage target.

In existing homes we use a pressure pan and blower door to test duct systems. In the low income weatherization programs we participate in, our pressure pan readings are measured at each supply and return register and have to be below 1 pascal. You can see a photo of one of our auditors holding a pressure pan over a supply register in an earlier image. The benefit of utilizing the blower door for testing is that we also get a measure of the homes building airtightness level.

What does it cost to seal a duct system?

Before rebates, a typical duct system that is accessible can be sealed and tested for $800 to $1,400. Our bid price depends on duct location, type of ductwork, duct area, and most importantly accessibility to ducts, which can vary from home to home. It will take longer to seal ducts in attics with a low pitched roofs or a tight crawl spaces. The mastic, duct board (if required) mesh tape, butyl tape, UL-181 approved tapes, disposable jump suits and gloves don’t cost nearly as much as the labor. We won’t provide a price on a duct seal until we have inspected the duct system. A typical duct system can be sealed in one day.

Duct sealing rebates

APS and SRP are offering a rebate as an incentive to seal ductwork. APS offers a $400 rebate that requires the duct system to be tested to verify duct leakage reduction. Starting February 1st, SRP offers a $250 rebate for single family detached homes. For all mobile home, townhome, apartment and condominium home types (duplex, triplex or quad plex only) there is a rebate of $200. The testing is a critical part of the job. The duct sealing rebate is a great value.

Energy savings from duct sealing

Energy savings depend on the severity of your climate and the extent of the duct leakage in your system. Fuel type also can have a significant impact as well. APS suggests in Phoenix that typical savings will be $200 a year. A DOE study says the range is somewhere between $100 and $400 a year depending on climate, fuel type and the amount of duct leakage measured. Another benefit of having your duct system sealed occurs during resale. Most home inspectors know enough to call out a duct system that is not sealed

Coupling duct sealing with duct modifications and insulation

At Advantage Home Performance we believe that duct sealing should go hand in hand with duct modifications and insulation repairs. Leaky, undersized and restricted ducts should all be repaired at the same time to achieve optimal HVAC system performance. The APS website addresses this issue of undersized returns. In the photos below you can see a flex duct deteriorating in the attic as well as a crushed and collapsed return duct. These are the kinds or repairs/modifications we are advocating for.


In the same homes with duct leaks, we often find insulation defects that also need to be repaired. As both a licensed HVAC and insulation contractor we know that duct sealing, duct modifications and insulation repairs are the most cost effective energy conservation repairs you can make to your home.


1. DOE – Measure Guidelines: Sealing and Insulating Ducts in Existing Homes

2. ACCA – The Indoor Environment & Energy Efficiency Association ACCA Standard 5: HVAC Quality Installation Specification.

3. Energy Star A guide to Energy Efficient Heating & Cooling

4. APS Website

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