Download PDF version (312.9k) Log In or Register to view the full article as a PDF document.

Pressure-Testing Ductwork, continued

The second step is to seal off the air distribution registers with a poly product called Duct Mask (Figure 3). It comes on a roll (also from the Energy Conservatory), so I simply run a belt through the roll and wear it around my waist. Duct Mask is perforated every 4 inches and has adhesive on one side. It makes sealing up the system such a simple task that I consider it essential. But I now have some mechanical contractors who seal all the supply registers that are installed in floors with sheet metal and mastic. This keeps construction debris and rain out of the ductwork and also prepares the system for testing.



Figure 3.The author carries a roll of adhesive poly Duct Mask on his belt (top) and uses the material to seal registers and grilles (bottom).

The last piece of equipment to hook up is the digital manometer, which is simple to set up and operate. The digital manometer I use both measures the pressure in the duct system and directly displays air flow through the Duct Blaster fan in cubic feet per minute, which is convenient and cuts down on errors.

I can often tell how leaky a duct system is simply by turning on the Duct Blaster and seeing how quickly or slowly the duct reference pressures respond. A leaky system will require that I ramp up my Duct Blaster fan speed to overcome the numerous leaks in the system. In fact, in some homes the ducts are so leaky that I can't pressurize the air distribution system at all. In a system with a return that leaky, it's very possible that more air is being drawn into the system from the crawlspace or attic than is being pulled from the house.

Once the air distribution system is pressurized to 25 pascals, I read the air flow through the Duct Blaster fan from my digital manometer.

It's important to recognize that the Duct Blaster measures the air leakage at a test pressure of 25 pascals, not the actual duct leakage when the system is running. The actual duct leakage under normal use depends on where the leaks are located and what pressures they see. The closer the duct leaks are to the air handler fan, the higher the pressures. The typical leaks where refrigerant lines enter the coil, for example, will see much higher pressures than the leaks around the supply boots at the end of branch runs, and thus will leak more air.

The tests are performed at a uniform pressure of 25 pascals because that represents a typical average operating pressure in residential systems and gives us a quick way to compare the measured leakage rate to accepted standards.

The Fog Machine

The shortcoming of a digital manometer readout is that tradespeople can't picture the leakage; in their minds, that number is not associated with anything. Also, we still don't know just where the leaks are. By introducing theatrical fog into the system through the Duct Blaster, we can make most of the leaks visible (Figure 4).




Figure 4.When a system exceeds the allowable air leakage standard, the author uses a theatrical fog generator to introduce visible vapor into the Duct Blaster fan intake. The fog makes it easy to find the leak locations.

The fog pouring out of duct leaks is a real eye opener. No one argues with this part of the test. Even after ten years of using a fog machine out in the field, I'm shocked sometimes that so much fog can leak out so fast. It is truly telling.

In my experience, installers really appreciate being on hand when their work is tested and fogged. It's often the first time they've seen their work tested, and the fog drives home the importance of paying attention.

Sealing the system. Fortunately, duct leakage problems are simple to eliminate during new construction if you know where to look for leaks and how to seal them. The cost varies depending on the system's size and complexity, but most systems can be sealed for somewhere between $150 and $600. Research has shown that the cost is recovered within one to five years; after that, the savings go to the homeowner's bottom line. And of course, the greatest benefit is a more healthful and comfortable home.

The products of choice are water-based mastic, accompanied by a fiberglass mesh on larger holes. The mastic has the consistency of mashed potatoes. It is easily spread over joints in the ductwork with an inexpensive paintbrush or one's hand. On cracks and gaps wider than 1/4 inch, a 2-inch fiberglass mesh tape is placed in a bed of mastic to reinforce the seal. The advantage of mastic over duct tape is that it provides a long-term durable seal. The water-based mastic cleans up easily with water.

Duct Tightness Standards

The goal of duct sealing is to ensure that the system is sufficiently airtight. That does not mean submarine airtight or hermetically sealed. We aim not to eliminate leakage entirely (although some contractors come close) but to reduce the leakage to an acceptable threshold. I use one of two standards for duct system airtightness, and each generates a "not to exceed" duct leakage number.

One standard is 3% of the livable square footage. For example, a 2,000-square-foot home should have no more than 60 cfm of leakage in the air distribution system. The other common standard is based on the size of the air-conditioning unit, which you can read off the unit's label (or the installer can tell you). It's common practice to assume 400 cfm per ton, so a 4-ton unit would be rated at 1,600 cfm; the air leakage should not exceed 5% of the total airflow capacity, or in this example, 80 cfm.

These levels are realistic on the job; conscientious workers can easily satisfy them. In my experience, if a crew can't meet these criteria routinely, either they don't have enough training and experience, or they aren't trying hard enough.

Who Should Test?

There are a growing number of third-party testing companies like mine. A list of testing contractors by state is available on the Energy Conservatory website (

I believe that hvac companies should conduct in-house testing of air distribution systems. If your hvac contractor already owns a Duct Blaster and other diagnostic tools, that tells you that the company truly wants to do quality work. Companies that don't own a Duct Blaster have to ask someone else to test their work, as well as show them their mistakes.

I've worked with architects who make the builder pay for the first test and the hvac contractor pay for the second test if he fails the first one. If a system fails, as a part of my fee, I'll spend up to an hour on the job helping the crew find the leaks with my Duct Blaster and fog machine.

Reaping the Benefit

On most jobs, there is a tug of war between quality and profit, and profit routinely wins. This means ductwork is often not sealed, or it's sealed but not tested. When customers have a comfort, air quality, or utility bill problem, it adds to their distress to learn that no one bothered to implement a simple quality-control step. They begin to wonder what other corners the builder cut.

I always advise my builder and hvac customers to at least offer sealed ductwork and testing. If you offer it and the customer turns you down, there's a record, in case of later complaints, that the customer chose low price over quality.

For builders who do test, we file a test report, which becomes a record for the house. I encourage my builder customers to use this report as evidence that they are truly committed to quality behind the drywall. If you seal and test ductwork and the contractor across the street doesn't, you have a better product, and you can use our documentation to build your reputation.

Satisfied customers also provide a marketing advantage. A customer who moves into a new home that is more healthful, comfortable, and energy efficient is more likely to tell friends that you are someone who builds a home that works.

Michael Uniackeruns Advanced Insulation, Inc., an Arizona company that supplies insulation, energy auditing, diagnostics, testing, and consulting services.

Sources of Supply for Duct-Sealing Mastic

AM Conservation Group, Inc.


Biddle Company


Carlisle Coatings & Waterproofing, Inc.


Carrier Aeroseal, LLC


(aerosol duct sealing franchiser)

Ductmate Industries


Duro Dyne Corporation


Foster Products Corporation


McGill AirSeal Corporation


Mon-Eco Industries, Inc.


RCD Corporation


Rectorseal Corporation


Shelter Supply