Leaky ducts do not announce themselves. The system runs, air comes out of the registers, and nothing sounds broken. The clues are in the house: one room that never matches the rest, dust that comes back days after you clean, a system that runs longer every summer. In attic-duct homes across the San Gabriel Valley and San Fernando Valley, leaks are the normal condition, not the exception.
Why attic ducts leak
Ducts are assembled from pieces. Every joint, collar, takeoff, and boot is a place air can escape, and the attic is hard on all of them.
In the 1950s through the 1980s ranch houses that fill Reseda, Northridge, and the newer parts of the San Gabriel Valley, the joints were sealed with cloth-backed tape. Summer after summer at 130 degrees dried the adhesive and the tape fell off. The flex duct that replaced some of those runs has its own failure points: the inner liner pulls off the metal collar when someone crawls past it, and the outer jacket rots in the heat until it splits.
Then there are the boots, the metal boxes behind each register. They are supposed to be sealed to the drywall. They usually are not, so a register is pulling and pushing air into the wall cavity or attic around its own edges. And rodents. A rat that finds warm flex duct in January chews in.
The Department of Energy's home upgrades guidance puts it plainly: in a typical house, about 20% to 30% of the air moving through the duct system is lost to leaks, holes, and poorly connected ducts.
The signs, room by room
One room never cools or heats. The room at the end of the longest run, the room over the garage, the addition somebody put on a Pasadena, CA bungalow in the 1970s. The duct feeding it is longest, has the most joints, and often runs through the hottest part of the attic or the garage ceiling. It loses air and picks up heat the whole way.
Weak airflow at some registers, strong at others. Hold a tissue at each register. If one barely moves it while the one in the hallway pins it flat, the air is going somewhere between the two.
Dust that comes back fast. A leak on the return side sucks attic air, and attic dust, into the system. It then gets pushed out every supply register in the house. Gray fuzz on the register louvers and a film on furniture a few days after dusting are the classic tell. Insulation fibers on the registers are a stronger one.
A hot-attic or musty smell when the system starts. Same cause. The first air out of the registers came from the attic, not the return grille.
Longer run times every year. The system takes longer to reach setpoint on the same kind of day it handled fine a few summers ago. During a heat wave it runs all afternoon without getting there.
Doors that slam or whistle when the system starts. Leaky ducts throw off the pressure balance between rooms. A bedroom door that swings shut on its own when the blower kicks on, or air hissing under a closed door, points at an imbalance somewhere in the system.
Dirty streaks around registers. A dark halo on the ceiling or wall around a register is dust being deposited by air leaking around the boot rather than through the grille.
Supply leaks and return leaks are different problems
A supply leak pushes cooled air into the attic. You paid to cool it and it is gone. The room at the end of that run gets less than it should.
A return leak pulls attic air into the system. In summer that air is 130 degrees and dusty, and the coil has to cool it before it can cool anything from the house. In winter it is cold. Return leaks also carry whatever is in the space they draw from. A return duct with a leak in a garage ceiling can pull in car exhaust and fumes, which is a safety matter, not a comfort one.
Most houses have both. Return leaks tend to matter more per square inch because the return side runs at higher suction.
What you can check yourself
- The thermometer test. Read the air temperature at the register nearest the air handler and at the farthest one, a few minutes into a cycle. A spread of more than a few degrees means loss along the way.
- The tissue test. Compare airflow register to register.
- Pull a register grille. Two screws. Look at the gap between the metal boot and the drywall. Look inside the boot for dust buildup and for daylight or insulation visible through a gap.
- Look at the filter slot. If there is a gap around the filter or the cover, that is a return leak right at the air handler, and it is pulling unfiltered air.
- Look from the attic hatch with a flashlight. Disconnected ducts, ducts lying on the attic floor, dark streaks at collars and joints, flex duct with a split jacket.
- Feel the air handler seams. With the system running, hold your hand along the cabinet seams and where the plenum meets the unit. Air hissing out or pulling in at a seam is a leak at the worst possible spot.
- Do not tape anything with cloth duct tape. It is the reason the ducts leak now. Do not walk on drywall. Do not disturb old paper-faced wrap or white tape on metal ducts, which may contain asbestos and belongs to a separate licensed trade.
When to call a licensed tech
- You want the actual number. A duct leakage test pressurizes the system and measures how much escapes, and it tells you whether the leak is at the plenum, in the runs, or at the boots.
- A duct has come off a collar in the attic.
- The return runs through the garage.
- The ducts show rodent damage.
- You are replacing the AC or furnace. California requires a leakage test with most equipment replacements, and the new system's airflow will not perform on ducts that lose a quarter of it.
- The far room has never worked and you want it to.
Sealing and repairing the ducts you have is duct repair. If the runs are undersized or the material is failing, our ductwork page explains when replacement is the honest answer. A leakage check is also part of a thorough HVAC maintenance visit.
Common questions
Can I seal ducts myself?
Mastic and foil tape on the boots you can reach from inside the house, yes. The joints that matter most are at the plenum and takeoffs in the attic, and the ones you can comfortably reach are rarely the big ones. Seal what you can, then test.
Does duct cleaning fix leaks?
No. Cleaning removes what is inside the duct. Sealing closes the openings. A cleaning sometimes reveals leaks, and a system that keeps refilling with dust after cleaning has a return leak somewhere.
How much leakage is acceptable?
California's energy code holds new duct systems to a low single-digit percentage of airflow, with a looser limit for existing systems being altered. The Department of Energy's estimate for a typical house sits far above that. Anything you can feel with your hand at a seam is too much.
If one room in your house has never matched the rest, we do a $69 diagnostic, credited toward the work, and give you a written quote before any work. Boost Services Heating and Cooling answers at 818-277-5056 between 6am and 10pm, seven days, or contact us online.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.