How much air do leaky ducts lose?
ENERGY STAR estimates that about 20 to 30 percent of the air moving through a typical home duct system is lost to leaks, holes and poorly connected ducts. In a hot-humid climate the consequences are worse than the percentage suggests, because supply leaks dump already-cooled air into an attic while return leaks pull hot, humid attic air into the system. Measured work in Florida homes found return leakage averaging around 21 percent of system airflow, and found that running the air handler raised whole-house infiltration roughly tenfold.
The short version
- ENERGY STAR: 20 to 30 percent of the air moving through a typical duct system is lost to leaks.
- Supply leaks waste cooled air. Return leaks are worse — they import hot, humid attic air into the house.
- Florida measurements found return leakage averaging about 21 percent of airflow across the homes studied.
- Running the air handler took whole-house infiltration from 0.14 to 1.42 air changes per hour in those homes. Repairing the return plenum brought it back to 0.31.
- 85 percent of the added load from that infiltration was latent — moisture, not heat. That is the humidity you feel.
- Measured work found a 4.6 percent return leak from a 120°F attic can cut cooling by 16 percent.
Two kinds of hole, two different problems
A duct system has a pressurised side and a suction side, and a hole in each behaves completely differently.
A supply leak is the intuitive one. The fan pushes conditioned air out through the gap and into the attic. You paid to cool that air and it is now cooling your roof structure. It is waste, and it is straightforward.
A return leak is the one that does the real damage in Florida, and it is counterintuitive because nothing is escaping. The return side is under negative pressure, so a hole there pulls air in — and in an attic, the air it pulls in is around 120°F and thoroughly humid. That air goes across your coil, into your house, and onto your energy bill as a moisture load your system now has to remove.
What was actually measured
The clearest work on this was done in Florida. Cummings and Tooley measured infiltration in Florida homes using tracer gas, with the air handler off and then on. With the fan off, the houses averaged 0.14 air changes per hour. With it on, 1.42 — roughly ten times as much air moving through the building envelope, simply because the duct system was running.
After repairing the return plenum in one case, the air-handler-on figure dropped from 1.42 to 0.31. The return leak fraction they measured averaged 21.6 percent of system airflow before repair.
The line that matters most for anyone living in Florida is this one: 85 percent of the added load from that infiltration was latent heat. Not temperature — moisture. That is why a house with leaky returns feels clammy even when the thermostat says it has hit the setpoint.
Later work by Withers and Cummings quantified how little leakage it takes: a 4.6 percent return leak drawing from an attic at peak conditions of 120°F produced a 16 percent reduction in cooling. Under five percent leakage, sixteen percent of your cooling.
Why you cannot just hand-seal it
Some of it you can. Plenum connections, accessible joints, the air handler cabinet — those are reachable, and mastic on a reachable joint is good work.
The problem is that most duct leakage is not at the places a person can reach. It is at connections buried in blown insulation, runs threaded through interior walls, boots sealed to drywall from the other side, and joints above a ceiling with no access. You cannot seal what you cannot touch.
This is the gap Aeroseal was built for. The system is sealed at the registers and pressurised, and an aerosolised sealant is introduced into the airstream. Air escaping through a leak carries the particles to the edges of the gap, where they accumulate and bridge it. The sealant goes wherever the air is going, which means it finds leaks by definition rather than by inspection.
It also measures leakage before and after, so the result is a number rather than a reassurance.
The part that gets misdiagnosed
The Department of Energy makes a point worth repeating: in the southern United States, attic duct leakage is often the actual cause of inadequate cooling, and it regularly gets mistaken for an undersized air conditioner.
That mistake is expensive in both directions. You pay for equipment you did not need, and the new equipment pushes more air into the same leaky system — so a larger share of a larger volume ends up in the attic. The house is no more comfortable and the bill is higher.
Before anyone replaces a system for capacity reasons in a Florida house with attic ductwork, the leakage is worth measuring.
What the measurements show
Common questions
How do I know if my ducts leak?
Symptoms worth noticing: rooms that never reach setpoint, a house that feels humid even at temperature, dust that reappears quickly after cleaning, and bills out of line with neighbours in similar houses. None of those is proof — leakage is measured, not guessed, which is why we measure it before and after.
Is 20 to 30 percent really typical?
That is ENERGY STAR's figure for typical homes, and the Florida return-side measurements are consistent with it. It sounds high until you consider how many joints a duct system has and how few of them were ever sealed with anything more durable than cloth tape.
Does sealing help with humidity or just energy?
In a humid climate, humidity is arguably the bigger win. Since most of the added load from return leakage is latent, cutting the leakage cuts the moisture your system is fighting. People often report the house feeling different before they notice anything on the bill.
Can sealing fix a system that was never sized right?
No, and we will say so. Sealing recovers air that is being lost. If the system was undersized or the ductwork badly designed from the start, sealing improves it without solving it. The inspection is what tells us which situation you are in.
What about the energy savings figures I see quoted?
There are credible national-laboratory figures for sealing attic and crawlspace ducts, and we cite them on the Aeroseal page with their source. What we will not do is apply a national average to your house and present it as your result. Your savings depend on your leakage, which is why we measure it.
Sources and standards
Where this page relies on published guidance, here is what it relies on.
- ENERGY STAR (US EPA / US DOE) — Duct Sealing — Federal estimate of how much conditioned air a typical duct system loses.
- Cummings & Tooley — Infiltration and Pressure Differences Induced by Forced Air Systems in Florida Residences (ASHRAE Transactions, 1989) — Tracer-gas measurements of return leakage and infiltration in Florida homes.
- Withers & Cummings — Air Handlers: An Appliance of Airtight Defiance? (ACEEE, 2006) — Measured effect of small return-side leaks drawing hot attic air into the system.
- US DOE / NREL — Better Duct Systems for Home Heating and Cooling — Department of Energy guidance on duct design, leakage and humidity.
