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How do you seal leaky HVAC ductwork?

Updated August 2026

2–3 hours · Easy DIY

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Brush duct mastic or apply UL-181 rated foil tape over every joint you can reach: 2 to 3 hours and $20 to $50 in materials for a first pass. ENERGY STAR puts typical duct leakage at 20% to 30% of the air moving through the system, and most of that escapes into an attic or crawl space where it does nobody any good. Cloth duct tape is the one thing to avoid, since the adhesive dries out and the tape peels within a year or two.

What you need

Before you start: The hazard is the space, not the sealant. Attics reach 130 to 150°F by midday in summer, so work early morning; step only on joists or a laid plank, because a foot through the ceiling is a $500 to $1,500 drywall repair; and wear an N95, gloves, and long sleeves against fiberglass. Watch for knob-and-tube wiring in older attics and stay clear of it. If ducts in a pre-1980 house are wrapped in gray-white fabric or crumbling paper, stop: that can contain asbestos, a lab test is $50 to $150, and removal is specialist work. Never seal over a combustion air opening or a return grille in a furnace closet.

Steps

  1. Set the fan to on at the thermostat so the ducts stay pressurized while you hunt.
  2. Walk every accessible run in the attic, basement, crawl space, and garage. Hold the back of a bare hand along seams, elbows, takeoffs, and the plenum, or light a stick of incense and watch which way the smoke pulls.
  3. Mark each leak with a strip of masking tape as you find it, then seal in one pass afterward. Hunting and sealing at once is how half of them get missed.
  4. Wipe every joint clean and dry. Mastic will not bond to a dusty duct, and neither will foil tape.
  5. Screw loose joints together with three evenly spaced sheet metal screws before sealing. Mastic and tape are sealants, not fasteners.
  6. Brush mastic about 1/16 inch thick and 2 inches wide over each seam and joint. Where a gap is wider than 1/8 inch, press fiberglass mesh tape into the wet mastic and coat over the top.
  7. On flex duct, pull the inner liner over the metal collar and band it with a nylon strap, mastic the collar joint, then pull the insulation and outer jacket back over and band that separately.
  8. Seal the register boots where they meet drywall or subfloor, working from below. Those boot gaps are usually the largest single loss in a house and the most commonly skipped.
  9. Let the mastic cure 24 to 48 hours, then check airflow at the farthest register. The rooms that never kept up in July tell you whether it worked.

Or hire it out

A contractor hand-sealing accessible ducts charges $500 to $1,200. Aeroseal, which sprays sealant inside the pressurized duct system and reaches joints buried in walls, runs $1,000 to $3,000 and includes before-and-after leakage numbers. Check your utility for current program money first. The federal 25C energy-efficiency credit ended on December 31, 2025, so do not build a budget around it.

Done. Now the part everyone skips: doing it again on schedule. Log it in OnOtto and the next reminder for duct sealing sets itself.

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Frequently asked questions

Why can I not use duct tape on ducts?

The name is a historical accident. Standard cloth-backed duct tape uses a rubber adhesive that bakes out in an attic and peels off the metal within a year or two, often leaving a gummy residue that makes the next repair harder. Foil tape carrying a UL-181 or UL-181B listing is rated for the job, and mastic outlasts both because it stays flexible and moves with the duct. A gallon tub of mastic is $15 to $30 and seals an average house.

How much will duct sealing actually save?

It depends on where the ducts run. A system whose ducts sit in a conditioned basement leaks air back into the house, so sealing mostly improves room-to-room balance. A system in a 140°F attic or a vented crawl space is dumping conditioned air outside, and sealing there cuts heating and cooling costs while fixing the bedroom that never cools. Comfort is the change most people notice first, well ahead of the bill.

Where do ducts leak the most?

Four places, in order. Register boots where they meet the floor or ceiling, because the drywall gap around them is rarely sealed. Takeoffs where a branch leaves the trunk. The plenum seams at the air handler, which see the highest pressure in the system. And every flex duct connection, where the inner liner works loose from the collar over time. Seal those four and you capture most of the leakage without crawling into the worst spots.

Should I have the ducts cleaned too?

Cleaning and sealing are different jobs with different payoffs. Cleaning pulls dust out of the ducts and earns its keep after renovation work, a rodent problem, or visible mold, but the EPA does not recommend it on a routine schedule. Sealing changes how the system performs. If you pick one, seal first. Duct cleaning intervals, inspection intervals, sealing intervals, and ductwork lifespan cover the schedule.

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