What do leaky, neglected air ducts cost every year?
Updated August 2026
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Leaky ductwork quietly throws away 20–30% of the air your furnace and AC work to condition, roughly $200–500 every year on an average $150–300 monthly bill. ENERGY STAR puts the typical loss in that 20–30% band, which means a fifth to a third of what you pay to heat and cool ends up in the attic, crawl space, or wall cavities instead of your rooms. Sealing the joints you can reach with a tub of mastic is a $50 afternoon; a professional aeroseal job runs $1,000–3,000 and usually pays for itself within a few heating and cooling seasons. Left alone, the waste never stops: it reappears on twelve bills a year, every year, while some rooms never hit the number on the thermostat. Here is what the leak costs at each stage.
The escalation ladder
Every row below is the same leak, left a little longer. The top is a cheap warning; the bottom is a claim.
| What happens | Typical bill | How fast |
|---|---|---|
| You seal the joints you can reach with mastic | $5–10 a tub, about $50 for the whole job DIY | One afternoon |
| A pro air-seals the whole system (aeroseal) | $1,000–3,000, often paid back in a few seasons | Half a day |
| You ignore it and keep paying the leak | $200–500 a year in heated and cooled air lost | Every year, indefinitely |
| Leaks pull dust and humid air from the attic or crawl | $150–400 duct cleaning plus dust and allergy complaints | Ongoing |
| Weak airflow gets "solved" by oversizing the next unit | Hundreds to thousands on a bigger furnace or AC than needed | At replacement |
Warning signs
Leaky ducts announce themselves as comfort problems long before you tie them to the bill. The classic tell is a room or two that never reaches the thermostat setting no matter how long the system runs, while the rooms nearest the air handler stay comfortable. Watch also for registers with weak airflow, a system that runs almost constantly in summer or winter, visible dust that reappears within a day of cleaning, and a musty or dusty smell when the fan kicks on, all signs the ducts are drawing unconditioned attic or crawl-space air. In an unfinished basement, attic, or crawl space you can often see it directly: gaps at the joints, sections hanging loose, and cloth duct tape peeling off the seams. Any of these means you are paying to condition space you never occupy.
The prevention math
The habit is nearly free: a $5–10 tub of duct mastic and one afternoon spent brushing it over every reachable joint, elbow, and boot in the attic, basement, or crawl space, plus foil tape on the larger seams. That $50 of materials goes straight against the $200–500 a year the leaks cost, so it can pay back the first winter. What you cannot reach, hire out as an aeroseal job for $1,000–3,000, still a bargain against a lifetime of the leak. Check the ducts each time you swap the filter and inspect them fully every year or two. See how often to seal air ducts and how often to inspect ductwork, and keep the filter fresh so the sealed system is not fighting a clog. Run your own numbers in the home maintenance cost calculator.
Every line in that table starts with a skipped 10-minute task. OnOtto keeps the 10-minute version on your calendar so the $2,000 version never happens.
Get the nudge before the billFrequently asked questions
How much can sealing my ducts lower my energy bill?
With a system losing the ENERGY STAR-cited 20–30% to leaks, sealing the worst joints can trim roughly 10–20% off heating and cooling, about $200–500 a year on an average bill. The savings scale with how much duct sits in unconditioned space: runs in a hot attic or vented crawl waste far more than ducts inside the heated envelope. A $50 DIY mastic job on the reachable joints captures much of that by itself.
Can I seal ducts myself, or do I need a pro?
Every joint you can physically reach seals with brush-on mastic and foil tape for about $50, and that alone catches a large share of the loss. The buried runs inside walls and sealed chases are where a pro aeroseal job earns its $1,000–3,000, fogging an aerosol sealant through the ducts to close gaps from the inside. One rule: never use cloth duct tape on ducts. Despite the name, it dries out and peels within a few years.
Do leaky ducts hurt anything besides my bill?
Yes. Ducts under pressure pull air from wherever they leak, so an attic return draws in dust, insulation fibers, and summer heat, while a crawl-space leak pulls moisture and musty odors indoors. You get rooms that never balance, dust that returns a day after cleaning, and worse allergy symptoms. Severe return leaks can also depressurize a mechanical room and contribute to backdrafting a gas water heater or furnace, so treat any carbon-monoxide alarm as a reason to call a pro.
How do I find duct leaks without special tools?
Run the system and hold a hand near the joints, elbows, and boots: you will feel air escaping at a bad seam, and you may spot disconnected sections or dried-out tape. The comfort clues count too, rooms that lag the thermostat and registers with weak flow. For a hard number, an HVAC contractor runs a duct-blaster test for $250–500 that measures total leakage and pinpoints the worst spots before you pay to seal.