Duct Sealing in San Mateo, CA

Duct sealing in San Mateo homes

Dust films the registers again a week after cleaning. One back bedroom never catches up. The furnace kicks on and the crawlspace odor arrives with it. Those three symptoms usually point the same direction: the duct system is breathing air it was never meant to breathe.

Leaks sit in predictable places here. In the 1940s–60s ranches, it's the boot-to-subfloor joint and the longitudinal snaplock seams on original galvanized trunk-and-branch. Gravity-furnace conversions leak at the oversized reused trunk takeoffs. Attic flex on 1980s-and-later additions pulls off the collar and sags, so the system drinks attic air. Near the shoreline, metal thins at the seams from salt. Remodel crews paint boots shut, a different problem, but it shows up in the same inspection.

Sealing is surface prep, not tape. Dusty galvanized sheds foil tape within a season, and cloth duct tape fails faster. Metal gets wiped, fastened mechanically, then mastic over mesh at wide joints. A duct pressurization fan gives before-and-after numbers; California ties duct leakage testing to equipment changeouts, so ask for the reading.

Some runs aren't worth sealing. Crushed flex, crumbling interior liner, rodent-fouled subfloor duct. And if the return is undersized, sealing the supply side first just raises static pressure.