Duct mastic
Also called: mastic sealant, duct sealant, HVAC mastic, UL 181 mastic
A thick, paintable, water-based sealant brushed over duct joints, collars, boots and plenum seams. It stays flexible and bonds permanently, unlike cloth duct tape, which dries out and falls off in an attic. A gallon is $20–$35 and seals a house.
ENERGY STAR estimates that 20–30% of the air moving through a typical duct system is lost through leaks, and most of those leaks are at joints: where flex duct meets a collar, where a collar meets the plenum, where a boot meets the drywall, and along plenum seams. Mastic is the sealant for all of them. It’s applied with a cheap brush or a gloved hand about as thick as a nickel, over fiberglass mesh tape for any gap wider than 1/8 in, and cures in a day.
The rules: seal the inner liner to the collar before strapping it, seal metal seams and collar flanges, and don’t rely on the outer jacket or tape to make a joint airtight. Ordinary gray cloth duct tape is not listed for ductwork and is the reason most old systems leak — it’s fine for the jacket, useless for the joint. UL 181B-FX foil tape is listed and works on clean, smooth sheet metal, but not on the rough, dusty surfaces most connections have.
Mastic is also what a duct-leakage test is checking. Many jurisdictions require the test at final inspection on a whole-house replacement, with a target like 4 CFM25 per 100 sq ft of floor area to outside; a system sealed with mastic passes, one sealed with tape usually doesn’t.