Duct Sealing in Sunnyvale, CA

One room never cools, and the attic gets the air instead

The symptom is usually a weak register. You hold a hand to it and feel almost nothing. In Sunnyvale tracts from the 1950s through the 70s, the trunk is galvanized with round branch takeoffs. The cloth tape on those old joints dries out and lets go. Later flex retrofits fail differently. A run gets crushed behind a truss, or the boot pulls free of the register. Conditioned air then vents into attic insulation.

Return-side leaks are worse for your lungs. A loose return joint pulls attic dust straight past the filter. During wildfire season that includes fine ash already settled upstairs. Gray streaking at register edges is the usual tell. Pull one register and look down at the boot collar.

Sealing means mastic, mesh tape at seams, and straps that hold mechanically. Tape alone is not a seal. Uninsulated metal boots also sweat on cool summer nights with the AC running, so those get wrapped.

Sometimes sealing is the wrong call. Deteriorated flex shedding gray liner fiber should be replaced, not patched. Same after rodent entry in crawlspace runs. Re-check joints after any re-roof, attic insulation job, or remodel.