Airflow testing is more commonly performed in commercial-industrial buildings, but is seldom done in homes, where IAQ can in fact be much worse. (Photo courtesy of Carrier Aeroseal.)
Recent reports have estimated that, on average, 90 percent of all residential duct systems have significant loss of airflow. Losses are caused by system leaks, such as poor seals and damage to sheet metal or ductboard ductwork, and damage and restrictions found in flex duct systems.

The ramifications in terms of wasted energy and system performance are staggering. The opportunities for HVAC contractors are huge, according to Mark Modera, Ph.D., P.E., who spoke at the 2004 Winter Meeting of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE).

When Modera addressed homeowners at the 2004 ASHRAE Public Session, he said, "Most of what you have heard is, there's a box, and there's a building. What about the ducts?"

"Do your ducts leak?" asked Modera, who is with Carrier Aeroseal in Piedmont, Calif. "The typical attic or crawl space duct system leaks 30 to 35 percent in the Sunbelt," he said. This particularly affects comfort on the hottest days. The energy savings potential runs from 15 percent to 35 percent, Modera estimated.

Forty percent to 60 percent of typical basement ducts leak, he continued, and there is more return than supply leakage; specifically, there is more leakage in panned-joist returns. This results in uneven cooling and heating, with an energy savings potential of 10 percent, Modera said.

All together, Modera estimated that in the Sunbelt (the Southeast and Southwest), 85 percent of the primarily flex duct or ductboard ductwork leaks. In the Frost Belt (upper Midwest and New England states), 100 percent of the primarily sheet metal ductwork leaks, according to Modera.

Problems with duct leakage often stem from poor installations practices. This leak may as well have the customer’s money, comfort, and goodwill pouring out of it.


Duct leakage and its efficiency ramifications are nothing new to contractors like Fred Seed, president of Arizona Air Balance Co., Tempe, Ariz. He has been involved in air testing and balancing since 1977, and has owned this company since 1987. "We sometimes test for $10 [million] to $15 million homes," he said. The company's services are priced for the commercial-industrial market.

"The gist of our work is extensive field measurements," he said. "Not many homeowners are willing to pay $500 to $600 for the report alone."

Does that mean the opportunity isn't there for residential airflow testing and balancing? By no means, implied Seed; it just means his company doesn't target that sector, except as part of the specs for those multimillion-dollar homes - and sometimes to provide empirical evidence in lawsuits involving tract homes.

There are similarities between commercial and residential airflow problems, he pointed out. The problems involving ductwork often result from poor installation practices. "Duct joints are not sealed properly," he pointed out, primarily due to the way it's sealed, more than the product being used. "It's more application than material," he said. Sealant may not be applied thick enough, or not thoroughly enough - if it's applied at all. "This is primarily what we would see in [the tract] homes," Seed said.

This agrees with findings of the Florida Solar Energy Center (FSEC) study, "Achieving Airtight Ducts in Manufactured Housing." The report cited these areas as the most common for airflow losses:

  • Leaky supply and return plenums.

  • Misalignment of components.

  • Free-hand cutting of holes in ductboard and sheet metal.

  • Insufficient connection area at joints.

  • Mastic applied to dirty surfaces.

  • Insufficient mastic coverage.

  • Mastic applied to some joints and not others.

  • Loose strapping on flex-duct connections.

  • Incomplete tabbing of fittings.

  • Improperly applied tape.

    Carrier Aeroseal’s duct-sealing technology can help seal holes up to 3/8 inches wide, after larger duct breaches have been repaired.


    The study made the following duct system recommendations:

  • Achieve duct tightness by properly applying tapes and sealing joints with mastic.

  • Accurately cut holes for duct connections.

  • Fully bend all tabs on collar and boot connections.

  • Trim and tighten zip ties with a strapping tool.

  • Provide return air pathways from bedrooms to main living areas.

    Of course, these tasks are less costly to perform before the house is completed. However, they are not impossible to achieve in retrofit situations. Because of this, the California Energy Commission is mandating airflow measurement and repair as one of its energy options for all residential retrofits in Title 24, which will go into effect in January 2005.

    Why is it being mandated? So it will be done, Modera said. It's one thing to say, "You should seal your ducts," he said. "That's nice. You should also call your mother." If it's still true that California leads the country in environmental regulations, look for this one to spread.

    In manufactured houses, at least, "Duct tightness goals can be achieved with minimal added cost," the study concludes. "Reported costs range from $4 to $8 [per house]. These costs include in-plant quality control procedures critical to meeting duct tightness goals."

    This brings up the issue of quality control during construction and during a retrofit.

    Checks And Balances

    Besides the obvious differences in system complexities, the main difference between commercial and residential airflow testing is when it takes place. Most testing services Seed performs in commercial-industrial projects take place before the building is handed over to the owners, at the commissioning phase. Most residential buildings are not commissioned at all - except, perhaps, for those very expensive ones.

    "Effective design for leakage means testing for leakage," Modera said. In California, one study found energy savings increased 15 percent with duct system testing. Without testing - when it was assumed that people had learned what to do - there was only a savings of 4 percent, Modera said.

    "You can't fix what you don't measure," stated Modera. "Most duct systems leak."

    John Proctor, P.E., is the principal of Proctor Engineering Group in San Rafael, Calif. "Residential and small commercial air conditioning rated efficiencies aren't automatic," he said at the ASHRAE Public Session. In order for homeowners to get the efficiency their unit was designed to produce, they need to be installed by quality contractors.

    Will all contractors be the best? No, he said. Will better-quality contractors cost more? Yes, because they need to maintain a higher profit margin and hire better-quality technicians.

    Proctor pointed out that in one study of residential air conditioning systems, 95 percent of them had failures in duct efficiencies (leakage), airflow (typically low), and refrigerant charge.

    Out of 55,462 field tests, low airflow was a widespread problem. "Efficiency is affected by low airflow," he pointed out. It can reduce furnace efficiency and cause premature heat exchanger and compressor failures.

    Commercial rooftop units had low airflow 63 percent of the time, he added.

    "Quality assurance is really important," Proctor said. It can range from the technician having a jobsite checklist, to having someone in the office check the technician's measurements, to having a third party check the measurements over the phone.

    "Variable-speed equipment is capable of going down to very low airflow rates," said airflow-testing contractor Seed. "Any air distribution problem is aggravated when the variable-speed system goes into low-airflow operation. For instance, if the home already has hot spots and cold spots, those problems are exacerbated by low-airflow problems. Other potential problems made worse include insufficient fresh air intake and excess humidity, depending on the climate."

    So, when does it make sense for a residential customer to have leaky ducts repaired? It depends on many factors, including the customer's priorities and living conditions, according to Seed. "Residences have the worst IAQ of any building," he said, due to careless cleaning, extreme amounts of cleaning, cooking fumes, pets, smoking, and relying on open doors and windows for fresh air.

    Poor airflow can directly affect comfort levels and energy efficiency, and can indirectly affect occupant health and medical bills.

    "Thorough testing and balancing will define duct-leakage problems, amount of fresh air intake, control, unit capacity, and IAQ, excluding mold," he said.

    Yes, there are opportunities for contractors in the residential market, Seed confirmed. Contractors looking for more information may want to contact the Associated Air Balance Council (, a nonprofit association of independent test and balance agencies.

    Publication date: 03/01/2004