What's the best product to use to seal ductwork?
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Our home is in Boston, MA. The ductwork in the basement is exposed and easily accessible. Basement is unheated and unfinished and gets damp in the spring and summer. Ductwork on the 2nd and 3rd floors is not accessible, and I don't expect to do any work on it. The previous owners sealed some of the ducts with duct tape, and wrapped the other part of it in standard insulation. Most of the duct tape is peeling off, and much of the insulation, attached with what looks like simple packing
Whether in new construction or a retrofit situation such as yours, the answer is the same: mastic duct sealant.
Metallic duct tape is notorious for coming loose over time. I have seen plenty of older homes with the tape literally falling off. Mastic has a much better track record.
Because it is elastic, mastic is able to expand and contract with the ductwork. Mastic also holds up well to condensation, whereas moisture can break down the adhesive bond in duct tape. Mastic is available at your local hardware store, and is easy to install, although this job requires diligence and patience.
You can also selectively remove the insulation, because it is probably hiding other seams that need resealing. Sealing the ducts is the number one priority, but when that is done, reinstall as much insulation as possible and install new green insulation if necessary. Tape is usually used to fasten the insulation in place, but a UL-181 listed tape (the stuff with the red lettering) should hold up better than the cheap stuff, and certainly better than packing tape.
This is a great project, and well worth the effort. Good luck!
For more information:
Read Rick Goyette's Q&A "Should I replace metal duct in a 1950s home with the current flex duct? Or is cleaning better?"