As long as ductwork has carried heated or cooled air to a space, there have been problems with air leakage. However, with energy prices spiraling upward, more focus is being paid to this industry-wide problem — especially since some estimate that residential and commercial duct leakage costs consumers about $5 billion a year. In addition, it is estimated that a typical house with ducts located in the attic or crawl space wastes about 20% of heating-cooling energy through duct leaks and draws approximately 1 kW more electricity during peak cooling periods.

In a 1998 study at Lawrence Berkeley National Laboratory (LBNL), researchers determined that duct tape is one of the main reasons why ducts leak. And duct tape is often used to seal ductwork connections.

As a result, one company has found itself a great niche: It offers duct sealing and repair services to help customers save on their energy bills while they reduce indoor air quality (IAQ) and comfort problems.

Why Tape Doesn’t Work

For three months, LBNL researchers tested a variety of sealing materials — many kinds of duct tape, clear plastic tape, foil-backed tape, mastic, and injected aerosol sealant — under conditions similar to those encountered in installed hvac systems. Of all the products tested, only duct tape failed. In fact researchers said that “It failed reliably and often quite catastrophically.”

In the tests, hot air (167?F) was forced through four sets of ductwork, and cold air (53.6?) through another four sets; the hot and cold airflows were alternated every 5 min. A bake test was also performed, in which sample joints were baked at temperatures of 140? to 187?. This simulated the conditions for air conditioning units and duct systems installed in attics.

Only one duct tape product survived three months of the aging test; 11 failed within days. Some fell right off the joint. Clear tapes, foil-backed tapes, mastics, and aerosol sealant, although they lack strength, formed good seals for the duration. While five duct tape products survived the baking test, in some cases this was because the backing separated from the glue, then fortuitously slid over the holes, plugged them, and baked shut again. In both kinds of tests, duct tapes — the majority of the products tested — were the only sealants that failed.

This is a problem that Michael Gamst, Ted Guzman, and Clayton Johnson, owners of Las Vegas Air Conditioning, Las Vegas, NV, see every day. “There’s a particular problem with flex duct, because when it’s put together with duct tape, the duct tape (especially in hot attics) will fail, and that’s when the ducts become leaky.

“I’ve seen it as soon as five years; you go up and the duct tape will be brittle, and there’ll be no stick left on the back of it,” says Guzman. Sheet metal itself is a problem too; it’s naturally leaky due to its many seams.

When leaky ductwork is encountered during a routine service visit, one of Las Vegas Air Conditioning’s specialized technicians offers to come back at a later date to perform a comprehensive IAQ evaluation for the homeowner. The evaluation is free.

Great Returns

For the last 18 months, Las Vegas Air Conditioning has offered the IAQ evaluation to customers who have experienced such problems as uncomfortable rooms, excessive dust, or foul odors. On routine service calls, the techs look for potential problems with ductwork that could cause IAQ or comfort problems. If they notice any, the techs recommend to the homeowner that the evaluation be performed.

The free evaluation takes approximately 2 hrs. According to Gamst, “There’s a risk, but you can gauge it over time and see if you’re losing money. Generally people are concerned. They’ve almost prequalified themselves when they ask you to come out to do the diagnostic, because [that means] they do have a concern. Most of the time we can fix whatever their problem is.”

The evaluation includes pressure testing the ductwork to determine the rate of leakage, measuring the cfm flows out of the registers, and also measuring register temperatures. The pressure testing is done either with a “Duct Blaster” or with “Aeroseal” diagnostic equipment. “We’ll also determine if the ducts are correctly sized by measuring the static pressure of the return and supply ducts,” says Guzman. “Because undersized ductwork is also an issue, and it will not do any good to seal the ducts if they are undersized.”

If there is a significant amount of duct leakage, the technician will recommend either hand sealing the ductwork with duct mastic or using Aeroseal, an injection-based aerosol, to plug up the holes.

“Basically we cover the return and supply openings, pressurize the ducts, and spray the aerosol into the airstream. It finds unwanted holes and plugs them, similar to ‘Fix-a-Flat’ on a tire. When you do that in conjunction with hand sealing the register boots and plenums, it’s really the best way to go,” says Johnson.

Aeroseal is a polymer that floats in the airstream. As the air goes through unwanted holes, it sticks to the sides; the holes get smaller and smaller until they’re completely plugged. An Aeroseal machine has a graph that shows the time vs. the leakage being sealed. Johnson notes that they usually seal down to 35 cfm.

Some homeowners may find the Aeroseal treatment a bit pricey. For a standard-sized house, Las Vegas Air Conditioning charges customers $800. However, Johnson notes that the service will more than pay for itself. “We’ve had customers who’ve saved up to $500 a year, although those are people who’ve had extremely bad problems. Generally $300 a year is a more realistic figure on average,” he says.

Duct Replacement

If a duct system is extremely old or leaky or made out of substandard materials, it may become necessary to rip out all the ductwork and replace it.

When it’s necessary to install a new duct system, Las Vegas Air uses duct mastic on every seam and connection, from the register boot to the flex duct connection to the plenum. The company pressure tests the system with a Duct Blaster when the installation is finished, to make sure the customer has as little leakage as possible.

Duct tape definitely has a role in this society — in fact, many would be lost (myself included) if it weren’t for duct tape coming to the rescue. But maybe it’s time for contractors to take the duct out of duct tape and start considering other options.

Sidebar: Duct Coatings for Accessories

Specifiers should exercise caution and judgment in the consideration of the coating requirements for sheet metal ductwork accessories, according to a technical paper prepared by John H. Stratton, director of Technical Services, Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA).

While the location and/or intended application for galvanized sheet metal ductwork usually dictates the requirement for G-60 and/or G-90 coating for the ductwork panels, similar coatings for the accessories may not be generally available in the normal supply channels. Special coatings for some accessories may be available at increased costs and delivery times.

Where ductwork exposures do not involve wet and dry cycles, high humidity, and chemically polluted environments, the traditionally used zinc coating amounts (reviewed in the paper) have provided satisfactory service life in hvac system service.

The service life of both G-60 and G-90 zinc coatings in outdoor exposures is relatively short. G-90 is a recommended minimum on exposed sheet and reinforcements when supplement paint coatings are not specified. The cost increase for G-90 vs. G-60 coated sheet is small but availability varies nationally.

The author asserts that any project specification that purports to make G-90 or G-60 coating a requirement for all steel components incorporated in ductwork and exposed to airflow is a radical departure from trade practice and will result in cost increases and procurement and delivery delays.

For more information, contact SMACNA at 703-803-2980; 703-803-3732 (fax); or visit (website).

Publication date: 10/17/2001