To help contractors as well as owners of residential hvac systems, new technological advancements may provide better duct-sealing solutions, particularly on existing systems where ducts are inaccessible.

At the same time, tougher state energy codes are mandating that new residential construction must meet duct tightness standards.

“Ordinary” or household-type duct tape isn’t up to the job, as a Lawrence Berkeley National Laboratory study pointed out (The News, Aug. 31, 1998).

“Our major conclusion so far is that you should use anything but duct tape to seal ducts,” one LBNL report states (defining duct tape as any fabric-based tape with rubber adhesive).

However, professional-grade tape, applied and combined with approved mastic and often using mechanical fasteners as well, can meet and sustain prescribed performance standards. Brushed-on interior coatings or aerosol mists sprayed into ductwork can also seal many leaks.

Documenting results

The success of both initial and remedial duct sealing and duct fitting can be documented with a lightweight, calibrated duct leakage testing system called the “Minneapolis Duct Blaster,” but often referred to simply as “Duct Blaster.”

Other instruments employed to measure and locate duct leaks include blower doors and pressure pans.

Despite recent revisions, UL Standard 181 is “still confusing,” says David Brook, energy specialist with the Oregon State University (OSU) Extension Energy Program. “They don’t really apply to sealing galvanized metal ductwork,” he says by way of example.

The OSU Extension Service offers contractors and other hvac professionals a free primer on Duct Sealing and Insulating to those who send a stamped (55 cents), self-addressed envelope to the Extension Energy Program, OSU, Batcheller Hall 344, Corvallis, Ore. 97331-2405.

The 10-page document, illustrated with line drawings, was funded by the U.S. Department of Energy and the Bonneville Power Administration and was first published in 1993.

Brook said the booklet is no longer distributed to the general public “because of concerns that without a sensitive manometer, do-it-yourselfers could unintentionally create potential backdrafting problems after sealing ducts.”

The new California residential energy manual, Chapter 2, depicts “well-constructed duct using diagrams from Uniform Mechanical Code (UMC) and the Air Diffusion Council.”

Chapter 7 of the same manual “includes criteria for obtaining energy credit for ducts constructed more efficiently than typical construction.” All the standards are effective for all building permit applications submitted on or after July 1 of this year.

The manual is retrievable on-line at the California Energy Commission Web page (

No one duct type leaks more

Frank Vigil, Building Science Specialist with Advanced Energy Corp., Raleigh, N.C., a not-for-profit organization, feels duct leakage problems are probably about equally distributed among major types of duct construction such as rigid, flex duct, or ductboard.

“It’s often a question of fabrication as well as durability of the material,” he says.

“In the majority of calls I receive where we do follow-up investigation, we find duct leakage as a contributor to health and safety issues, building durability, comfort, and energy concerns,” Vigil reports.

“Indoor air quality, moisture, and high bills are often the result — at least partially — of leaky ductwork, as well as thermal envelope and framing problems.”

Both UL Standards 181A (rigid ducts) and 181B (flexible ducts) anticipate new or different products and systems, saying that any that “involves a risk of fire, electric shock, or injury to persons, shall be evaluated” for safety.

One of the newest approaches to interior duct sealing is an aerosol-borne sealant, the weapon of choice for Aeroseal, Austin, Texas. (See accompanying article, page 14.)

U.S. Intelli-Coatings, Lucerne, Calif., is working on a from-the-inside application of its sealants using a self-propelled applicator, “the concept being similar to the cleaning devices in use by duct cleaners,” explains Keith Buter, co-owner with founder Ken Parlet II. They are pursuing testing to determine the suitability of the process for flex duct use.

The company already markets an exterior sealing “Duckt System” in which a primer and a sealant embedded in a polyester fabric are top-coated twice with an elastomeric paint in which ceramic oxide particles are suspended.

“By applying this coating to ductwork, you will have an airtight and watertight system and . . . add the insulative value of up to 14 in. of standard fiber glass insulation,” the company claims. “Past savings show the system paying for itself through energy savings in as short a time as six months,” Buter notes.

“Exposed, large sheet metal ducts outside of buildings insulated by conventional fiber glass or other polyurethane products are damaged by ultraviolet light,” U.S. Intelli-Coatings points out.

“Most outdoor insulating systems offer minimal insulation, and when exposed to the elements, break down very quickly, opening the system to leaks.

“Air leaking in or out can be expensive in wasted utility expense, but water leaks are an even more serious liability problem,” the California firm asserts.

Fiber glass-lined ductboard often breaks down and starts loosing fibers into the airstream, sometimes because of damage during or after installation, according to Safko. To cure that problem and further seal the duct system from inside, his firm applies a plastic coating (a polymer, Porter Paints #3830, from Courtauld’s Coatings, Louisville, Ky.) with a vehicle drawn through the installed ductwork.

For exterior sealing of ductwork, Safko uses either a linseed oil-based mastic (avoid adhesive-based mastics, which may outgas, he warns) or a foil-backed tape, Indoor/Outdoor Foilgrip duct sealer with butyl-based caulk, to wrap joints.

High standards

While standards for duct sealing are much more strict in current construction codes, he notes, “most contractors don’t do a thorough duct sealing with tape and mastic.”

To do the job to “healthy home” standards might add $500 to the cost of a new system, a cost that can be recovered quickly in lower heating and cooling expense, Safko says.

Even a remedial sealing application, however, can often be done in an average home for between $500 and $700, again recoverable in a short time with energy savings, Safko notes. (His e-mail address is INDOOR

One of the prime sites for system leakage and related problems is on the return air side of the system. In some parts of the country, building cavities are used as return ducts.

Wood framing, drywall, and plywood, frequently “panned” with sheet metal, don’t form airtight seals. “They are the most important place to start duct sealing,” according to Energy Outlet, Eugene, Ore., an electrical energy conservation center sponsored by three utilities (

Such building cavity plenums can easily pick up pollutants from attics, crawl spaces, or garages, and they’re easily and frequently punctured by plumbers, electricians, and homeowners because only the hvac contractor may know where they are.

“Seal such building cavities from inside them, whenever possible,” Energy Outlet advises.


Advanced Energy Corp. (introduced earlier in this article) trains contractors and their employees in duct sealing. “Eight and a half years ago, as a service to the hvac industry, we started teaching a course on ductwork diagnostics and repair,” says Frank Vigil.

“Response was good, and we thought that the need for such training would disappear within a couple of years. This turned out to not be the case.

“Only a very small percentage of contractors and technicians understands the impact the duct system has on occupant health, safety, and comfort, as well as building durability and energy efficiency.

“Many in the industry recognize duct leakage as an issue, but market forces (price competition) are such that addressing duct leakage aggressively hasn’t come into play yet. Acceptance of ‘standard practice,’ which includes leaky ductwork, is the norm,” Vigil laments.

“We have trained some 400 contractors in our certification program to date. Our programs run for six or 11 days, depending on whether the individual wants certification. The first six days are heavy lab and classroom with some field experience, and the next five days are spent largely in the field on actual construction, with about four trainees or students per course leader,” Vigil says. (The training schedule and articles of interest to hvac specialists is available at

Training alone isn’t always the answer, he cautions. Competition may help in some cases.

“A few years ago, we worked with an eastern North Carolina utility company to develop an energy conservation program with a duct air-tightness standard. Out of 200 homes built in the program in the first year, subsequent testing showed about 50% of the houses failed to meet the standard.

“With a slight amount of additional training, things changed the next year. Participating contractors were discovered to be competing with one another on who was getting the tightest duct systems.

“The program found just four duct airtightness failures [not 4%, but four houses] out of the same number of houses. The leakage measurements were not just within limits, but often was so low it couldn’t be measured.” (Vigil’s e-mail address is fvigil

Selling the service

At A.A.C. Inc., Mesa, Ariz., vice president Walt Sikora tellsThe Newsthat “only people who really want comfort” can be easily sold on checking the tightness of their duct systems.

On its website (coolnow. com), his firm cites the Yale study reporting that 95% of homes tested have significant duct leakage.

No utilities in his market are currently offering incentives for duct leakage reduction, Sikora says. Consequently, selling that service to customers interested in laying out only a minimal expenditure on their hvac system is tough.

“We do blower-door testing” to check for leakage, he reports. Professional-grade mastic is applied to joints and other leakage points. If the leaky duct is within a wall, a section of the wall must be removed to fix the leak.

“Some customers will have us do just a section of their system to see what a difference duct sealing can make,” he says. “Once they experience the results, they’ll call us back to do the rest.”

All Weather Heating & Air Conditioning, Jonesboro, Ga., tells visitors to its website (all that “an old, leaky duct system can lose 30% or more of the air you’ve paid to heat or cool.

“Sealing up those leaking joints (or replacing the duct system entirely) is an investment that pays off very quickly.”