Leaky ducts: the cooling you pay for and never receive

Updated August 25, 2026 · Claude AC · 3 sources cited

Every conversation about cooling efficiency starts at the equipment — SEER2 ratings, tonnage, brands. In a ducted American house, a large share of the loss happens after the machine has done its job correctly. The Department of Energy’s figure is the one worth memorizing: in a typical house, about 20 to 30 % of the air moving through the duct system is lost to leaks, holes and poorly connected joints. You paid to cool that air. It went into an attic.

This is why a new 17 SEER2 system can perform worse than the neighbour’s 14 SEER2 one, and why « it still doesn’t cool the back bedroom » survives an equipment replacement. Ducts are the delivery half of the system, and the half nobody inspects.

Where the air actually goes

Duct leakage is rarely one dramatic hole. It is the accumulation of ordinary construction details:

• Joints and takeoffs — every branch connection is a seam, and seams held only by a screw and a strip of tape open as the metal moves through thermal cycles.
• The plenum and the air handler cabinet — the highest-pressure point in the system, and often the leakiest. Losses here are the most expensive per square inch of gap.
• Flex duct connections — inner liner pulled over a collar, banded, then the insulation and outer jacket pulled over separately. Skip a band and the joint breathes.
• Building cavities used as ducts — panned floor joists and wall cavities serving as return paths in older houses. Never airtight, and the usual route by which attic or crawlspace air enters the living space.
• Boot-to-floor and boot-to-ceiling seams — the register boot is sealed to the duct but frequently not to the drywall or subfloor it passes through.

The consequence is not only lost capacity. Supply leaks pressurize the attic and depressurize the house, pulling unconditioned outdoor air in through every other gap in the envelope. Return leaks do it directly: the system inhales 130 °F attic air and tries to cool it, along with whatever moisture and insulation fibres come with it. A leaky return in a hot-humid climate is a humidity problem masquerading as a capacity problem.

Symptoms that point at ducts, not equipment

• Rooms far from the air handler never keep up, while rooms near it overcool.
• The system runs nearly continuously in mild weather that should not require it.
• Visible dust streaking at supply registers, or dust accumulating faster than the filter schedule explains.
• Rooms that are stuffy and humid despite the thermostat being satisfied.
• Ducts you can see in an attic or crawlspace: crushed flex, disconnected branches, sections of grey cloth-backed « duct tape » that have gone brittle and let go.

The last one is worth checking today, with a flashlight. A fully disconnected branch in an attic is commoner than it sounds, and produces exactly one symptom: a room that has never worked, usually blamed on the room rather than the duct.

Measuring rather than guessing

The professional test is a duct pressurization test: a calibrated fan sealed to the duct system, registers masked, leakage measured at a reference pressure and expressed as CFM25 — cubic feet per minute at 25 pascals — normalized against conditioned floor area. Modern energy codes set targets in that unit, and a house tested well above target has a quantified, fixable problem rather than a vague one.

A blower door test measures the whole envelope and, run alongside a duct test, says whether your leakage budget is spent on ducts or on the shell. Utility-sponsored energy audits often include both at little or no cost. Ask for the numbers on paper and for a post-work re-test — sealing without one is a promise, not a result.

What to seal with — and what never to use

The naming irony is genuine: the one product named after ducts is the one that fails on them. Laboratory durability testing pushed the trade toward mastic and UL 181 tapes, which is why inspectors look for a brushed-on grey seal rather than silver tape.

Insulation is the second half of the job: sealing a duct that then runs bare across an attic stops the air loss but not the heat gain through the duct wall. Supply ducts in unconditioned space are commonly specified at R-6 to R-8 in current codes. Seal first, insulate second — insulating over an unsealed joint hides it permanently.

Indicative costs

These are market reference bands for typical US residential work in 2026, not quotes — labour rates and attic access move all of them, and any number offered before someone has been in the attic is a sales figure. Utility and state programs frequently rebate duct sealing, because it is one of the cheapest kilowatt-hours a utility can avoid; check it alongside equipment incentives in our credits and rebates guide.

Ducts before equipment

The sequencing argument is routinely ignored. A load calculation performed on a house with 25 % duct leakage produces an inflated load, hence an oversized system that short-cycles, dehumidifies poorly and wears its compressor faster — and the leakage is still there afterwards, now moving more air into the attic per hour than before. Sealing first shrinks the load, and a smaller correctly sized system costs less to buy and less to run. If a contractor proposes a replacement without measuring or even mentioning the ducts, that is the question to ask before signing; the rest of the quote-reading checklist is in our replacement cost guide, and the sizing method that duct leakage distorts is in the BTU sizing guide.

The same logic explains why ductless equipment sidesteps the problem: a mini-split has no ducts to leak, which is much of why its rated efficiency survives contact with a real house. In an existing ducted home, sealing is almost always the better dollar — it fixes the rooms that never worked, cuts the dust reaching your filter, and compounds with every measure taken afterwards. Where it belongs in the yearly rhythm is in our maintenance guide: static pressure across the air handler is the reading that finally names a duct problem out loud.

Frequently asked questions

How much can duct sealing actually save?

Enough to matter, and highly variable. A house at the bad end of the DOE range — near 30 % of its airflow lost, ducts in a hot attic — can see a double-digit cut in cooling energy plus rooms that finally hold temperature. A house with ducts inside conditioned space saves little, because the leaked air never left the envelope. Duct location matters as much as the leakage rate.

Can I seal ducts myself?

The accessible ones, yes. Mastic, a disposable brush, UL 181 tape, a respirator and coveralls; work in the cool of the morning; seal every joint, seam and boot you can physically reach, including the air handler cabinet. Ducts buried in walls or ceilings need the aerosol method. Do not disturb anything resembling older insulation wrap without identifying it first.

Is aerosol duct sealing worth the extra cost?

It is worth it precisely when the leaks are inaccessible, which is when it gets proposed. Priced against opening ceilings it is the cheaper path, and it produces measured before-and-after leakage figures. Ask for those numbers in writing; they are the deliverable.

Should I seal the ducts or replace them?

Seal, unless the ducts are physically compromised — crushed flex, sagging runs with standing kinks, disconnected branches, undersized trunks, or building cavities pressed into service as returns. Sealing a bad duct system yields a tighter bad duct system; problems caused by geometry need geometry fixed.

Do leaky ducts affect indoor air quality?

Yes, and this is the argument that persuades people the energy numbers do not. Return-side leaks in an attic, crawlspace or garage pull in whatever is there — insulation fibres, humid air, dust, and in an attached garage, combustion by-products — and distribute it through the house at the rate of the blower. Sealing the return side is the highest-priority half of the job for that reason alone.

Read next

• Attic insulation and cooling: the load you pay to remove
• Upstairs hot, downstairs cold: fixing uneven cooling
• Cooling a garage or workshop: what works and what the code forbids
• Homeowners insurance and your air conditioner: what a claim actually pays
• R-454B and the A2L switch: what the refrigerant change means for you

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