How much do leaky ducts actually cost?

Typical homes lose a fifth or more of conditioned air through duct leakage, more when ducts run through attics and crawl spaces, which translates directly into runtime, bills, and rooms that never reach setpoint.

Should I use duct tape on ducts?

No, ironically. Cloth duct tape's adhesive fails on ducts within a few years. The materials that last are duct mastic brushed over joints and UL-181 rated foil tapes.

How can I tell if my ducts leak?

Feel joints with the blower running, look for dust streaks at seams and boots, note rooms that starve, and read the filter: fast-loading filters and musty supply smells point at return leaks pulling from attics and crawl spaces.

Are return duct leaks worse than supply leaks?

Often, yes: a leaky return in a crawl space or attic vacuums that space's air, dust, moisture, and worse, into the system and distributes it house-wide, an air-quality problem on top of the energy one.

Ductwork Leaks: Finding and Sealing Them

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Ductwork is the half of the HVAC system nobody inspects, and it leaks: at joints, boots, plenums, and the flexible runs stapled up decades ago, typically bleeding a fifth of every heating and cooling dollar into attics and crawl spaces. Duct leak sealing is the rare project that is cheap, DIY-accessible for exposed runs, and repaid in both comfort and bills. This guide covers finding the leaks, the materials that actually last, and the leaks that matter most.

Why ducts leak, and why it costs so much

Ducts assemble from sections, takeoffs, boots, and flex runs, and every connection is a candidate: original installs sealed with cloth duct tape whose adhesive retires in a few years, joints never sealed at all, flex duct torn by traffic or sagging off its collars, and boots that were never fastened to the floors they serve. The cost arrives three ways. Supply leaks dump already-conditioned air into spaces you do not live in. Return leaks vacuum those spaces instead, pulling attic heat, crawl-space moisture and must, insulation fibers, and soil gases into the airstream, the air-quality half of the problem, and the reason a fast-loading filter is a duct clue. And pressure imbalance from both makes rooms starve, doors slam, and the system run longer per degree, which is wear masquerading as weather. One safety note gives return leaks extra weight: a return leaking near combustion appliances can depressurize the mechanical room and invite backdrafting, which promotes duct sealing from thrift to safety.

Finding the leaks

Work with the blower running (fan ON at the thermostat) and the ducts exposed in basement, attic, or crawl space. Hands first: palm every joint, takeoff, and boot, feeling for jets, then let a smoke pencil or incense stick make small leaks visible where hands are unsure. Eyes second: dust streaks and gray fuzz radiating from a seam are leaks that have been filtering attic air for years, and disconnected or crushed flex runs announce themselves by geometry. Rooms testify third: the chronically cold bedroom at the end of a long run, the supply register barely breathing, the closet return whistling. And the professional version, a blower-door and duct-pressurization test, puts a number on total leakage, worth buying when planning bigger air-sealing and insulation work since the tests share a visit.

Sealing that lasts

The materials rule is famous for its irony: not duct tape. Duct mastic, a thick brushable paste, is the durable standard: brush it generously over every joint, seam, takeoff, and boot connection, bridging gaps up to about a quarter inch, with fiberglass mesh tape embedded across anything wider. UL-181 rated foil tape is the legitimate tape, good on clean straight seams and for clamping flex-duct inner liners to collars (foil tape plus a zip tie or clamp on the liner, insulation pulled back over and taped again). Fasten before sealing: pull disconnected joints back together with sheet-metal screws so mastic seals a joint rather than spanning a canyon. Prioritize by pressure and consequence: the plenum and first joints at the air handler (highest pressure, biggest leaks), return-side leaks in dirty spaces, boots at floors and ceilings (sealed to the surrounding surface, a famous bypass), and then outward along the runs. Insulating attic and crawl ducts after sealing compounds the win, and sealing must come first, since insulation over a leak just filters it.

What to leave alone, and the aerosol option

Panned joist returns and building cavities used as ducts, common in older houses, leak by construction and deserve professional conversion rather than heroic caulking. Interior wall runs are unreachable by brush, which is where professional aerosol sealing (injected sticky particles that bridge leaks from inside) earns its fee, verified by before-and-after leakage numbers. And any asbestos-suspect duct wrap on older systems is a stop-and-test, not a weekend project.

The bottom line

Ducts leak at every joint they were never sealed at, tape dies, mastic lives, and returns in dirty spaces are the leaks with teeth. An afternoon with a brush and a bucket on the exposed runs is the cheapest capacity upgrade the system will ever get, and the starving back bedroom is usually the first to say thanks.