Homefix: Dwight Barnett answers home-improvement questions. This week's topic is on repairing leaky duct work.

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Q: I had a heating-and-air-conditioning professional come to my home and do a home-energy analysis. He says my ductwork needs to be repaired upstairs and in the crawlspace, because a good deal of leakage is bringing bad air pollutants into my home. He wants to repair it with either mastic and tape or foam, which is more expensive ($2,500). Are either of these the better route to take as far as repairs? Is that a reasonable price for the foam? I have gone online and seen several different ways to repair ductwork, but I am not sure which is best.

A: Several companies are now offering a free or reduced-cost energy analysis as a means of selling a product or service. A company cannot stay in business by offering free services, so it finds a way to grab your attention and then it tries to get you to sign on the dotted line.

That said, the HVAC professional is correct in pointing out that leaky ducts are adding to the costs of heating or cooling your home. I checked some of my older residential-code books and found (starting in the 1983 edition) that ducts were required to be sealed at all joints and seams. Before the energy crisis of 1974, sealing of the ductwork at the time of installation was often omitted, owing to the added costs to the installer. Just a few years ago, I pointed out the leaky ducts in a new home during an inspection. When the builder asked the installer why the ducts were not sealed, he stated that he could not justify the added costs with the bid he had given the builder. In other words, corners were cut in order to get the job.

Yes, the ducts need to be sealed, but I’m not aware of a foam sealer. Any foam used on the ductwork needs to be fire-resistant, which is an added cost for the material. I recommend either a duct mastic or metallic tape to my clients, but not duct tape. Over time, duct tape will not hold up on the ductwork, and air leakage will occur. Sealing the ducts is expensive because it’s a labor-intensive job. The area of the ducts to be sealed has to be cleaned before adding the mastic or the tape, and the confined working conditions of a crawlspace or an attic only add to the time it takes to complete the work.

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Don’t forget to ask the installer if the costs include sealing the register openings, called a boot. Normally, the boot is caulked to the subfloor to prevent bypass air between the house and the crawlspace.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions d.Barnett@insightbb.com.

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