The best way to seal a leaking air duct is to clean the joint and apply HVAC mastic or UL-181 foil tape on accessible seams, not ordinary cloth duct tape. Most leaks form where ducts meet boots, plenums, or the air handler, and sealing these spots cuts air loss and improves comfort.
In our coastal service area, humidity can dry out old adhesives, so older foil or fabric tapes often fail before the metal does.
Try these focused steps on reachable sections:
- Turn the system off; wipe, vacuum, and dry metal surfaces.
- Brush on water-based mastic for gaps up to 1/4 inch; add fiberglass mesh over wider seams.
- Use UL-181 foil tape on straight seams and press firmly for full contact.
- Seal the boot-to-drywall gap with mastic or caulk and replace any missing screws.
- Rewrap disturbed insulation and avoid compressing it.
If a run is hidden, brittle, or in a tight attic, let a pro test and seal it safely.
Updated: 2026-06-18
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