Sealing follows testing. Once you know how much air a duct system is losing and roughly where, sealing is the repair: mastic and mesh at joints, proper mechanical fastening and sealing at takeoffs, and correcting the flex connections that have pulled loose or were never made properly to begin with.

The reason it matters on commercial work is scale. A leak rate that would be an annoyance on a house becomes real money on a system moving thousands of CFM, twelve or more hours a day, through a Florida cooling season that runs most of the year. And the loss is not only energy, it is also pressure, which is why sealing frequently fixes comfort complaints that no amount of thermostat adjustment ever did.

There is a compliance side too. On new construction the mechanical spec often sets a leakage threshold that has to be met and documented. Sealing to that threshold, then retesting to prove it, is what closes that item out.

Materials and workmanship account for most of it. Tape applied over dusty or oily metal releases. Mastic spread too thin cracks as the duct expands and contracts with each cycle. Flex duct clamped once at a boot but never checked for the inner liner sealing separately from the outer jacket will leak air and let humid plenum air into the insulation.

Movement is the second cause. Duct systems flex every time the fan starts, and a joint that was marginal on install day works itself open over a few thousand cycles. Add other trades working above the ceiling in an occupied building and connections get disturbed regularly.

Our climate then decides how much that costs you. Because so much commercial duct here runs through hot, humid unconditioned plenums and attics, a leak on the supply side dumps cold air where nobody benefits, and a leak on the return side pulls in outdoor-humidity air that the equipment has to dry before it can cool anything. Along the coast in Destin, Miramar Beach and the 30A towns, that return-side leakage is a common reason a building stays sticky.