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…