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How Can I Seal a Leaking Air Duct to Improve HVAC Efficiency?

You can seal small duct leaks using mastic sealant or UL 181 foil tape to boost HVAC efficiency. Do not use cloth “duct tape.” For large gaps or damaged flex, schedule professional duct repair.

Quick steps:

  • Run the fan and locate leaks at joints, boots, and seams.
  • Turn off power to work safely.
  • Clean surfaces; let dry.
  • Brush on mastic; add mesh for gaps > 1/8 inch.
  • Tape straight seams with UL 181 foil tape.
  • Re-insulate exposed ducts when cured.

Updated: 2025-12-24

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