As indoor air quality (IAQ) becomes a greater concern for everyone, the subject of ductwork leakage comes up more and more often.

Besides affecting air quality, excessive ductwork leakage can cause a myriad of other problems affecting air conditioning and heating capacity, efficiency, reliability, safety, and humidity control.

Abnormal ductwork leakage is fairly widespread across the United States. Based on studies of ductwork leakage done around the country, average duct leakage appears to be in the neighborhood of 17% of total airflow. No one knows what the actual amount is in any given area, but just the fact that any study could come up with these kinds of numbers anywhere is pretty startling.

What I want to talk about in this article is leaky ductwork installed outside the conditioned envelope of the structure. The conditioned envelope is the area of the building that is intended to be cooled and/or heated. Leaky ductwork that is installed inside the conditioned envelope, while creating some problems, is not nearly as serious a condition.

Whether a system has supply leaks or return leaks often makes little difference in the degradation of system performance and IAQ.


Obviously, if the return air duct system is installed in/or is utilizing the infrastructure of the building (outside the envelope), air will likely be drawn from the attic or other unconditioned space. What is not so obvious is that supply leaks cause the exact same problem.

When a structure has supply leaks, the air-handling equipment is drawing more air out of the envelope than it is returning to the envelope. This creates a negative pressure in the conditioned space, which causes air to be pulled in from outside the envelope.

Of course, any air that comes in from outside the envelope is not being filtered. Much of the air will come from outdoors if the structure is “loose,” which is bad enough, but if the structure was built tight or it has been tightened, air will be drawn from the infrastructure.

Drawing air from the infrastructure is worse than from the outdoors because the air is often of lower quality than outside air.

  • The air may have to move through insulation to gain access to the envelope, contaminating the conditioned space with fiberglass or other material, such as mold.

  • Some buildings have bathroom exhaust fans that terminate in the attic. That could spoil your dinner.

  • Not only is the air from the infrastructure contaminated, but in the cooling season it is often hotter than outside air.

    Any way you look at it, supply and return leaks have a negative impact on the performance of the equipment and the indoor air quality.

    So how can you tell if a structure has leaky ductwork? Probably the best way is to perform a blower door test. However, this test is fairly involved; many service techs and companies are not equipped or trained to perform them.

    Some other techniques, although not as effective, can help reveal the presence of ductwork leaks using tools that are common (or should be) to many shops.


    But if you already know that a supply and return ductwork installation is not sealed properly, there is no need to troubleshoot it. You will only confirm what you already know: It leaks.

    The most effective way to diminish leaks in this type of ductwork system is to seal the likely sources. For supply and return leaks, the obvious thing to do is to seal all the joints in the ductwork, including the air handler or furnace.

    Some leak sources are less obvious. A very common source of supply leaks is where the register boot penetrates the envelope. The register boot opening should be sealed at the penetration so that when the supply air hits the back of the register, it cannot deflect back into the infrastructure.

    Another source of leaks is caused when the infrastructure itself is used as the ductwork. A common example of this is when the supply or return air is moved through the structure between unsealed wall stud or floor joist cavities. We would all like to think this is OK, but when you actually measure the amount of leakage from these installations, it will astound you.

    Another common source of air leakage is from unsealed return pedestals (platforms). I’m talking about the installations where the air handler or furnace is sitting on a return pedestal in a closet within the living space. If you look under the platform, you can often see wall studs. You can bet money that a lot of air is being drawn from outside the envelope via those wall cavities.

    Finally, I once found a house that was getting return air from around the fireplace flue cap area. The implications are obvious.


    There are some relatively easy ways to determine if a structure has significant ductwork leaks. To do this, you need to be able to measure the airflow through the ductwork system in cubic feet per minute (cfm). To measure cfm you will need a flow hood. Many shops don’t have a flow hood, so let’s talk about how to solve this problem.

    You will need, at the very least, a velocity meter, preferably digital. Digital velocity meters measure the velocity of airflow, are available for as little as $100, and are more than accurate enough to serve the needs of field service and installation people under most circumstances.

    Figure 1 shows an example of the one I use. Digital velocity meters measure the velocity of airflow in feet per minute (fpm). Although cfm is what is needed to evaluate airflow, there is an easy and inexpensive way to get these velocity meters to measure in cfm. All you have to do is build an airflow hood (Figure 1). The one I’m currently using is made out of a U-Haul cardboard moving box. Of course it can be made out of plastic or duct board for more durability (and to avoid customer perception problems). I’ve tried making them out of sheet metal, but they are just too heavy to use.

    Anyway, one end has to be 12- by 12-in.-sq inside dimension; the other can be as large as you want. If you place this flow hood over a grill and measure the velocity of the air in fpm as it flows through the opening, the fpm readout will be the same as the cfm flowing through the opening.

    Technically the face area of the 12- by 12-in. opening should be sized at 144 sq in. plus the face area of the velocity meter you are using. Voila, you have a digital flow hood.

    Don’t get me wrong; I’m not saying this Rube Goldberg flow hood is as good as a store-bought version. I’m saying that it is accurate enough to diagnose 95% of the problems encountered in residential and light commercial installations. If you have a professionally built flow hood, by all means use it.

    Let’s talk about how to use

    our new Hybrid Cardboard Box Thingamajig (HCBT). When measuring the airflow through the flow hood, place it over a register with the 12- by 12-in. end away from the register. The air moving through the register will now be flowing through the flow hood.

    Leaving the velocity meter in the off position, place it in the airstream, allowing the propeller to come up to speed. Turn the velocity meter on (it already should have been set to average fpm mode). The velocity meter will immediately begin to average the velocity of the airflow through the hood. Traverse the velocity meter across the opening of the hood to get an accurate reading of the average airflow through the entire face of the hood opening.

    As you can see in Figure 1, I mount my velocity meter on a stick so that my hand will not add restriction to the 12- by 12-in. area. This will give you the velocity, and more importantly, the cfm flowing through the grill.

    1. Before beginning your duct leakage tests, seal any intentional fresh air openings to prevent them from misleading you into thinking they are return leaks.

    2. With the air handler operating, measure the airflow out of each supply register; total the readings to get total supply airflow.

    3. Measure the airflow into each return register; total these readings to determine total return airflow.

    If the total airflow of the supply system is close to the total airflow of the return system, the ductwork is very likely intact.

    An additional benefit of this test is that you now have a good idea of what the total airflow is through the system.

    If there is a significant difference between the return total and the supply total, there is duct or air handler leakage.

    If the return total is more than the supply total, the system has dominant supply leakage. If the return total is less than the supply total, the system has dominant return leakage.

    What I mean by dominant leakage is that both the return and the supply sides could be leaking, but one of the sides has leaks that dominate over the other side. For instance, the return side could have 200 cfm of leaks and the supply could have 100 cfm of leaks. Inside the envelope this would show as a 100-cfm dominant return leak. (See Figure 2.)

    What this boils down to is that if you detect a difference between the total supply air and total return air, all the air could be leaking in one side or the other, but not necessarily.

    In addition, the amount of leakage measured in the envelope can look significantly less than the actual leakage. If you seal all the accessible leak sources and retest, you will be able to tell if you still have leaks in the remainder of the system.


    You can take this test even further, if necessary, to isolate the leak sources.

    You could disconnect the return ductwork from the air handler, turn the blower on, measure the airflow entering the return opening, and compare it to the supply air leaving the supply grills to confirm supply leakage. In this scenario, you would have to induce a restriction on the return opening that matches the restriction of the existing return ductwork. This could be accomplished by matching the return pressure to its pressure prior to removing the return ductwork.

    You could also confirm return leakage by reversing this process. One note of caution: It’s important to remember that there is a possibility of equal leakage in the supply and the return. In this case, you would not detect airflow differential between supply and return.

    Usually it is not necessary to go to the lengths just mentioned because if you have sealed those leak sources you can access, you have very likely solved the majority of leak problems that would exist on most jobs. But it’s nice to have the knowledge necessary to dig into those structures that are more challenging.


    At the beginning of this article, I mentioned that safety was also a concern when there is duct leakage. As we discussed earlier, if an envelope has a dominant supply leak, it will have a negative pressure on it when the blower is operating.

    If the same envelope has gas appliances in it that have inadequate combustion air inlet systems, the negative pressure in the envelope can cause these appliances to generate potentially deadly carbon monoxide. One of the implications of this is that a structure should not be left with a dominant supply leak.

    A repair to the return air system without ensuring that the supply was also sealed properly could create this situation. Ensuring that your customer’s gas appliances are ventilated properly will also minimize this possibility.

    Howard Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

    Publication date: 05/13/2002