Duct tape is reportedly good for everything from hemming pants to insulating the insides of boots.

But how good is it at sealing air ducts in forced-air heating and cooling systems? And how does one determine if those ducts need to be sealed in the first place and, if so, where?

You may be hearing more questions like these: Duct testing and sealing has been a hot topic among energy geeks for years, but recent code changes are pushing the subject into the mainstream.

The Problem

Ducts can leak as much as 25% of the air they carry. If they run through an unconditioned space, as in most homes, those leaks are a problem. “A leaky return in an attic can suck 140-degree air into the system during summer and 30-degree air during winter, making the furnace and air conditioner work harder and burn more fuel. The homeowner ends up paying to heat and cool the outdoors,” says Steve Easley, a California-based construction consultant specializing in solving problems related to building science.

Easley adds that high energy use isn’t the only consequence of leaky ductwork. Others include:

An uncomfortable home. Because the optimal amount of air is not being delivered to the right spaces at the right volume, unwanted hot and cold spots can be created in the house.

Mold and moisture. In summer, humidity pulled into an air-conditioned house through duct leaks can condense on cold wall surfaces, leading to mold and mildew. In winter, humid air leaking from a duct into a cold attic can condense on the roof sheathing, turning to ice, then eventually melting and wetting the attic insulation, leading to mold growth and structural rot.

Life safety. Some leaks can create a slight vacuum in the home. If the home has a tightly sealed building envelope and open combustion appliances, that could, in extreme cases, cause combustion byproducts such as carbon monoxide to backdraft into the home.

It’s best to place all ducts inside the home’s conditioned space; otherwise, ducts outside the conditioned space need to be insulated and air sealed.

The requirement to insulate and seal ducts was written into the 2012 International Energy Conservation Code (IECC). The requirement—indeed the code itself—has met with strong resistance from builder groups, but some states, including California and Illinois, have fully adopted it. Some builders in other states are meeting or exceeding the code’s requirements as a way to differentiate themselves in the local market.

In jurisdictions that have adopted these requirements, ducts completely inside the envelope need no insulation. However, those running through the attic need a minimum of R-8 insulation, while ducts in other unconditioned spaces need R-6 or better.

Testing for Leaks

Total system air leakage must not exceed 4 cubic feet per minute for every 100 square feet of living space. This has to be verified at a test pressure of 25 pascals, which approximates the typical pressure in a residential forced-air system. The tool for testing leaks is called a duct blaster, which includes a calibrated fan for measuring airflow, a pressure sensor, and a microprocessor.

Using a duct blaster is quite simple. The tester uses tape, cardboard, or special vent caps to seal all but one of the home’s supply and return air registers, then connects the duct blaster fan to the open register. When the fan is turned on and the test pressure reached, the sensor records the total air leakage in the ductwork, and the microprocessor uses this to calculate whole-house leakage.

If total leakage exceeds the code’s requirements (or the contractor’s target rate), then someone has to find and plug those leaks. “Most leaks occur at boots, elbows, and where one duct section connects to another, so one way is to simply feel around those connections for moving air,” says Easley.

Testers also use a smoke pencil, which is a handheld, battery-powered device that generates puffs of “smoke” (actually a mix of water vapor, glycol, and glycerin) that can be used to locate moving air. Another approach, says Easley, is to pump theatrical smoke into the ductwork and look for where it leaks out.

The Fix

What’s the best way to seal these leaks? Building America offers simple guides to duct sealing details as a webpage at its Solution Center website and as a downloadable pdf file. Most HVAC and air sealing pros seal most joints on rigid duct with mastic. On flex duct, they often use a combination of mastic and tape.

The type of tape counts. Conventional duct tape is really misnamed, as it’s a poor choice for duct seams because its rubber-based adhesive tends to dry out and fall off after a while. It can also give off toxic smoke when burned, which is why the state of California prohibits its use on ducts.

The IECC requires that any tape used for duct sealing be certified to meet the UL 181A or 181B standard. The standard tests include things like temperature, flame, corrosion, and puncture resistance, all of which are important. However, the tests do not predict the longevity of the seal.

There are reports that a few newer UL-listed tapes offer better long-term performance because of the types of adhesives and backings they use. They include some oriented polypropylene tapes, which resemble packing tape and have an acrylic adhesive, and some butyl adhesive tapes. The latter include foil-backed tapes (which look like aluminum foil) and some with a cloth backing.

The generic gray stuff still has its uses, but those tend to be for things like taping up cardboard boxes or lining those boots—if you believe that works. —Charles Wardell

“A leaky return in an attic can suck 140 degree air into the system during summer and 30 degree air during winter, making the furnace and air conditioner work harder and burn more fuel.”