Leaky ducts in a forced-air heating and cooling system are an all-too-common problem contributing to significant energy losses and lower indoor air quality.
Mark Renfrow knows that. Duct tests at his 3,400-sq. ft. home revealed “huge leakage.” A contractor addressed the problem by applying mastic to any accessible ductwork. But the key word is “accessible.” Many parts of the system apparently are not so easy to reach.
“We retested and got down to about 25% leakage, and at that point the contractor said it was the best they could do,” Renfrow writes in a Q&A post at GreenBuildingAdvisor.
“The ductwork is all metal with insulation,” he adds. “We have a lot of ductwork. Tearing all the old insulation off to seal the joints seems excessive when I don’t really know what is causing the leakage. It my be in inaccessible runs most likely in supply areas.”
Renfrow has two questions. First, can ducts be sealed effectively from the inside? And second, how come mechanical contractors can’t do a better job of isolating the source of leaks?
Although the original post isn’t new, the topic continues to interest GBA readers and is the subject of this week’s Q&A Spotlight.
There are methods for isolating leaks
In reply, several posts suggest there are established methods for narrowing down the source of duct leaks. David Meiland, for example, writes that the air handler can be partitioned with cardboard and masking tape effectively enough to discover whether leaks are more serious on the supply or the return side. “You can also get a general idea of which side has more leakage when the blower door is set up, by running the air handler and watching what happens on the manometer,” he says.
Danny Kelly suggests using a blower door and pressure pan. “Run your blower door like you typically would and take a reading at each supply/return register,” he writes. “Since your ductwork is supposed to be a closed system, you should get a reading of 0 at each register in an ideal world. By doing this you can often find the few runs that have the most leakage and concentrate on those. An experienced BPI [Building Performance Institute] contractor can help with this.”
Art Vandelay agrees that a specialist should at least be able to determine whether the supply or return side of the system is the chief culprit. “Tape off all registers, then turn the [air handler] fan on for a brief period. Insert pressure probes in both supply and return plenums. Whichever side is at a lower operational pressure, as measured with manometer while the fan is ON, is leakier,” he writes. “Don’t leave the fan on for too long, and use care if the trunk is made of duct board.”
Where to get the right kind of help
One hurdle to improving the performance of forced-air systems is finding a professional who’s fully up to speed. Renfrow says he talked to two HVAC contractors who didn’t know how to isolate the problem. “In fact,” he says, “they shrugged their shoulders and said that much leakage is normal.”
“The average person can barely understand duct leakage and the cost implications,” Meiland writes. “As you have seen, many HVAC contractors aren’t up to date either. The typical energy auditor can barely sell their services, much less the upgrades a typical house needs, partly due to the short-term mentality that most homeowners have, along with the lack of ready money for improvements.”
Kelly suggests the searchable database at the Building Performance Institute. “I would look for one with the Envelope Professional designation,” Kelly says.
Also, he adds, it might be possible to find an expert through the National Comfort Institute.
The Aeroseal approach
Given the difficulty of removing duct insulation to seal all connections, tackling the problem from the inside of the system is appealing. The company mentioned in this thread is Aeroseal. According to the company’s website, the process was developed at the Lawrence Berkeley National Laboratory with funding from the U.S. Environmental Protection Agency. The technology is now licensed to Aeroseal.
Renfrow found a single contractor in the Dallas-Fort Worth area who uses the Aeroseal process and was given a rough estimate of $4,000. That brings a response from Brad Brenner, who works with the company. “The DOE has called Aeroseal technology one of the top 23 most important consumer technologies to come out since the agency was first established,” Benner writes. And while the technology has been available for a few years, he adds, it hasn’t received much attention.”
“A new company dedicated solely to Aeroseal technology was established earlier this year,” he says. “They are working on changing that. The first order of business is getting more contractors educated, trained, and licensed.”
According to Benner, the cost to an average homeowner will be between $1,100 and $2,000, not the $4,000 Renfrow was quoted. More important, the process typically reduces heating and cooling costs by about 30%, allowing homeowners to save between $250 and $850 a year on utility bills. “It’s a fairly quick and unobtrusive procedure,” he says, “in and out in a day.”
Our expert’s opinion
Here’s how GBA technical director Peter Yost sees it:
It’s really hard to pinpoint leak locations in duct systems. The tests described are all good for determining which side of the system has leaks, but not for locating them.
Aeroseal works; it is an effective interior sealing method for smaller leaks. But the Aeroseal application has trouble spanning and sealing big leaks (larger than 5/8 inch). Aeroseal literature indicates that their system is appropriate for duct systems with pre-seal testing of 12% leakage or more and post-seal testing of 8% or less.
Studies have shown that the biggest bang for the buck is meticulously sealing (with duct mastic and fiberglass mesh tape) the supply and return trunks as well as the plenum. Access is generally easy for these three.
There is a relatively new test, the Zone DeltaP test. In this test, “zone bags” are used to restrict air flow in different locations; when inflated, the bags create different pressure levels and air flows upstream and downstream of the bag. The results can target duct repair/sealing to the most leaky locations.