Sealing ducts is now recognized as being very important. In fact California, always a bell-weather state, is now requiring duct testing and sealing on most change-outs and new construction. Building and energy codes are popping up all over that now require tight ducts in new construction.

Why seal ducts? Research proves that typical systems lose 20-30 percent of their capacity and efficiency due to duct leaks, some much more. Duct leakage brings in significant summertime humidity in hot humid climates, contributing to mold growth, higher utility costs, and discomfort. A leaky duct system can't be balanced properly. Duct leakage also brings in dusty and potentially contaminated air from attics, garages, and crawlspaces. In some cases, duct leaks lead to CO poisoning or building rot.

Sealing and renovating ducts isn't as exciting as installing the latest high-efficiency equipment or IAQ accessory, but if pre-existing leaks are ignored, that new product simply won't deliver its potential. As Pacific Gas and Electric (PG&E) recently stated in a consumer protection booklet, it doesn't make sense to install a new, energy-efficient heating and/or air conditioning unit unless the duct system is also energy efficient.

HVAC replacement contractors across the nation are now embracing duct system performance testing and renovation as a value-added service that differentiates them from the 13 SEER pack. Whether you do it voluntarily or are forced into it, your company is going to need to get more proficient at selling and delivering duct-repair services.

Fiberglass ductboard is considered by many to be an excellent material for damming returns and lining chases.


Let's start by reviewing the many materials available for sealing ducts, identifying the best ones to use, and the ones not to use.

Fabric-backed duct tape: That old standby, grey fabric duct tape, used to be considered the obvious product for the job. Well, field experience and research testing has now proven that while this type of "duct tape" has many uses, sealing ducts just isn't one of them. It may take as little as a week or as long as 10 years, but when applied to ductwork, rubber adhesive fabric tape will eventually dry up, get brittle, and fail.

Department of Energy (DOE) scientists at Laurence Berkeley Laboratories (LBL) performed accelerated aging testing a few years back. "We tried as many different kinds of duct sealants as we could get our hands on. Of all the things we tested, only duct tape failed. It failed reliably and often quite catastrophically," said Max Sherman, who heads the LBL Energy Performance of Buildings Group.

Even clear, plastic carton-packing tape outperformed fabric duct tape. Simply put, don't use fabric duct tape to seal ducts.

Foil duct tape: Foil tapes with an acrylic adhesive usually significantly outperform fabric tapes. However, foil tape has limitations as a retrofit and repair product. The surfaces must be absolutely clean, which can be a challenge on a 20-year-old system in an attic or crawlspace. The product also doesn't conform well to sealing certain gaps, such as dovetails at start collars.

Nevertheless, it has its place when working with new, clean products, such as sealing the joint between a new furnace and an evaporator coil. Scrim-reinforced UL 181A-listed tape works well to fabricate new duct board systems, if it is applied properly with an iron and squeegee. To fixed failed duct board joints, the best solution is mastic with reinforcing mesh, not duct tape.

Butyl-backed foil tape: A superior product to the thin, acrylic-adhesive foil tape is one with a thicker (15 to 50 mil) backing of butyl adhesive. It seems much less sensitive to surface cleanliness, conforms better to irregular shapes, and sticks like crazy.

Often called "mastic tape," it combines the ease of taping with the durability of duct mastic. It works well to seal flex duct to collars, unions, and boots. It covers "S" and drive joints well. It also is a good way to seal the boot to the sheetrock joint, if slit into 1.5-inch strips. Although much more expensive than normal foil tape, it works well and lasts.

Fiber-reinforced mastic: This is the preferred material for manually sealing most duct leaks. It is a gooey adhesive that is applied wet, and it sticks to almost anything. The fiber-reinforced variety reliably bridges gaps up to about 0.25 inch and is less sensitive than tape to dirty surfaces. It dries to a soft solid, similar to a thick latex paint.

A paintbrush can be used to paint the mastic on the joint. In some cases, you'll be forced to smear the product on using your hand. (It is recommended that you wear a throwaway glove.)

When faced with wider gaps, or when a stronger mechanical connection is needed, use reinforcing-scrim mesh tape, such as drywall tape. You will find that mastic without fibers the consistency of yogurt brushes better when working with scrim.

On some gaps, first install a backer layer of fabric duct tape to keep the mastic from slumping through. Be sure to extend the mastic an inch on either side of the duct tape. If old duct tape is present, peel it off if it comes off easily. If not, encapsulate it with the mastic.

No matter how careful you are, mastic is pretty messy looking. Neater joint lines can be achieved where necessary by using painters' masking tape, peeled off before the mastic cures. On bare sheet metal, ask your supplier for a gray-pigmented mastic to blend in better.

For residential applications, use only the water-based variety, not one with a strong-smelling chemical solvent. Mastic comes in buckets of various sizes, and also in a caulking tube for more pinpoint installation.

Silicone caulking: Clear silicone caulking is a good choice to neatly seal joints at the equipment, both in cabinets as well as connections between furnaces, coil cases, and external filters. Drawbacks are that it has quite an odor, is somewhat time-consuming, and is expensive.

Fiberglass duct board: Even if you prefer metal for fabricating duct systems, duct board is one of the best products to have on hand when repairing an existing return system. Duct board is great for creating joist or stud cavity blocking, and for lining building cavity return chases or plenums. Duct board has a unique characteristic ("wedgibility") that no other acceptable product has.

Do not use building sheathing foam boards inside returns. They do not meet the fire and smoke requirements. Sheet metal and drywall could be used, but they are much more awkward and messy than plugs of duct board.

Internal sealants: A sticky vinyl polymer is applied to the leaks internally using a proprietary-franchised process ("Aeroseal" from Carrier Corp.). It is pumped through the duct system, where it seals leaks from the inside and dries. The process deposits the sealant droplets at the gaps and holes in the duct without actually coating the interior of the duct.

This process works best when sealing leaks under 3/8 inch across. Bigger leaks, and leaks right at the equipment, still need to be sealed by hand. If building cavities are used for returns, be careful when trying to pressurize these areas; they can leak badly and the sealant can make a mess. Sometimes hand sealing returns is a better option.


Be careful when you expand your business into duct sealing. "Duct renovation" is a better term for the work that usually needs to be performed, rather than duct sealing. Unless care is taken, sealing ducts can have unanticipated negative side effects. For example, after sealing duct leaks, static pressure usually goes up and airflow usually goes down. In extreme cases, this can lead to coil icing and furnace overheating.

Most systems have more problems than leaky ducts. You also need to test for good static pressure and airflow. Returns are a very common required add-on; complete replacement is often warranted.

In general, look at the interactive effects on the entire home. Sealing duct leaks often changes internal pressures, and may inadvertently cause combustion appliance back drafting, increased dust, or inadequate ventilation. A partial seal that leaves the house at an imbalanced air pressure can result in no energy savings.

Get comprehensive training on building science, duct design, duct renovation, and air balancing in order to best serve your customers and reduce liabilities and callbacks.

Publication date: 08/07/2006