Failed mastic or tape at joints
Cloth-backed tape and some mastic applications degrade over years of attic heat cycling, opening gaps at every joint they once sealed.
Duct Leakage
How Can I Tell if My Air Ducts Are Leaking?
You can't confirm duct leakage by feel alone. Suggestive signs include a hot attic that stays hot after the AC runs, rooms that never reach set temperature despite normal register airflow, unusually high bills for the system's age, and visible gaps, torn tape, or disconnected flex where you can see ductwork. A duct leakage test is the only way to quantify it.
Key takeaways
- Leakage does not reliably weaken register velocity, so airflow at the vent can feel fine while air is still being lost in between.
- Attics and crawlspaces are the places to look, not the rooms themselves.
- A pressurized duct leakage test is the only way to get an actual number.
- Some signs point to equipment or duct sizing instead — worth ruling out before assuming leakage.
Most likely causes
Boot-to-drywall or boot-to-subfloor gaps
The register boot is often attached to the framing but never sealed to the drywall or floor around it, leaking straight into the wall or floor cavity.
Disconnected or crushed flex duct
A flex run pulled loose at a takeoff, or crushed under attic storage or foot traffic, dumps air into the attic instead of the room.
Plenum and cabinet seams
Seams at the air handler cabinet and supply/return plenums are common leak points, often overlooked because they are close to the equipment.
Panned returns and building-cavity returns
Return paths built from floor joist bays or stud cavities instead of sealed metal or duct board pull in unconditioned attic, crawlspace or wall-cavity air.
What you can safely check yourself
- Feel around visible register boots and duct connections in the attic for a light stream of air on a running system.
- Look for torn, sagging, or disconnected tape and mastic on any accessible ductwork.
- Check whether the attic near ductwork feels noticeably hotter than the space it serves.
- Note whether rooms take a long time to reach set temperature despite normal-feeling airflow at the register.
- Look at utility bills relative to the system's age and the season for anything that stands out.
Stick to what you can see, feel and hear. Leave electrical panels, refrigerant, combustion appliances and equipment internals to a licensed professional.
What a professional should measure
- Total duct leakage test (pressurized duct blaster) measuring total system leakage.
- Duct leakage to outside test, isolating leaks that vent to unconditioned space from leaks within the envelope.
- Static pressure readings across the air handler and filter to flag other restrictions alongside leakage.
- Visual inspection of accessible duct runs, boots, plenums, and connections with a leak-detection tool.
- Room-by-room airflow measurement to see whether delivered air matches design.
When duct leakage could be involved
Duct leakage happens on both the supply and return sides, and the two behave differently. A supply leak loses conditioned air on its way to a room, so the room receives less air even though the register itself may still show reasonable velocity, because most of the pressure drop happens before the leak, not after it. A return leak pulls in unconditioned or contaminated air before it reaches the coil, diluting what the system is actually conditioning.
Leaks are also split by destination. Leakage to outside — into an attic, crawlspace, or exterior wall — is a straight loss of conditioned air and money. Leakage to inside — for example a supply leak into a wall cavity that still opens into the house — recovers some of that air but not in the room it was aimed at, which is why one room can run behind while overall bills still rise.
Because register velocity depends mainly on the pressure available at that outlet, a duct system can leak a meaningful share of its air upstream while every vent in the house still 'blows fine.' That is the main reason homeowners underestimate leakage from feel alone, and why a pressurized test is the only reliable measurement.
When this probably isn't a duct-sealing problem
- If airflow is weak at every register and the attic doesn't feel unusually hot, restriction (filter, coil, blower) is a more likely first check.
- If the whole house struggles evenly rather than specific rooms or a hot attic, equipment capacity or sizing may be the driver.
- If bills rose alongside a change in occupancy, thermostat settings, or weather, that may explain the increase without leakage.
When to call an HVAC contractor
Duct Daddy specialises in duct leakage testing and duct sealing. We do not perform mechanical HVAC repair, refrigerant work or equipment replacement. Contact a qualified HVAC contractor if any of the following apply:
- Airflow is weak at most or all registers regardless of duct condition.
- The system short-cycles, runs constantly, or trips a breaker.
- You suspect a refrigerant, electrical, or coil issue.
- Equipment age or performance suggests it may need service or replacement.
Why this matters in Southeast Louisiana
Most Southeast Louisiana homes route ductwork through hot, humid attics, which accelerates tape and mastic failure and makes leakage to outside especially common here.
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Frequently asked questions
- Can I see duct leaks without going into the attic?
- Rarely. Most leaks occur at joints, boots, and plenum seams that are only accessible from the attic or crawlspace, not from inside the finished rooms.
- Does a leaking duct always feel weak at the vent?
- No. Because leaks are usually upstream of the register, the vent can still show normal airflow while a portion of the air never arrives at all.
- Is a hissing sound proof of a leak?
- A hiss near a duct joint is a reasonable indicator, but it doesn't quantify how much air is escaping or whether it's going outside or into the house. A test provides the number.
- What number counts as leaky?
- Duct leakage is expressed as leakage to outside per unit of conditioned floor area or as a percentage of system airflow; there isn't a single homeowner threshold, which is why the test itself matters more than any rule of thumb.
Get an actual leakage number
Rather than guessing from clues, a duct leakage test measures exactly how much conditioned air your system is losing and where.
Related problems
Signs of leaky ductwork
The pattern of symptoms — comfort, bills, and physical clues — that together suggest duct leakage.
What Is a Duct Leakage Test?
What the test actually measures, how it differs from a general inspection, and what it does not tell you.
How Is Duct Leakage Tested?
The equipment and steps behind a duct pressurisation test, and what each reading measures.
Do leaky ducts raise bills?
Yes, leaking supply and return ducts can raise energy use — but the size of the effect depends on where the leaks are and how the ducts are routed.
What a Duct Leakage Number Means
How to read a CFM25 result — relative to what, judged against what, and what it can't tell you alone.
Browse every symptom in the troubleshooting center.
Find out what your duct system is actually doing.
Schedule a duct leakage evaluation and get measured numbers for your home — not a guess.