Duct leakage is conditioned air escaping through gaps in the duct system before it reaches a room. In homes with ducts in attics, crawlspaces or garages, that air is lost entirely. Leakage causes uneven temperatures, longer run times and higher bills, and sealing is usually cheaper than upgrading equipment.
Where ducts actually leak
Duct systems rarely fail in the middle of a straight run. They leak at every place two things were joined:
- Takeoffs where a branch leaves the main trunk
- Boots where a duct meets the floor or ceiling register
- Joints and seams in rigid duct, and the collars on flex duct
- The air handler cabinet itself, and the plenum connections at both ends
- Panned joist returns in older homes, where a cavity between floor joists was used as return duct — these are often very leaky by design
On the supply side a leak means less air reaches the far rooms. On the return side a leak means the system draws in whatever surrounds the duct — attic air in August, garage air with whatever is stored in there, crawlspace air with its dust and moisture — and sends it through the house. That is an indoor air quality problem on top of an efficiency one.
Signs your ducts are leaking
- Rooms far from the air handler never keep up, particularly upstairs
- The system runs much longer than it used to for the same comfort
- Dust reappears within days of cleaning, especially near registers
- Registers in distant rooms produce weak airflow even with a clean filter
- The attic or crawlspace feels noticeably conditioned near the ductwork
- Noticeable temperature difference between the air at the air handler and the air at the farthest register
A rough check you can do yourself
Turn the fan to ON so air is moving. Then go to any accessible ductwork — basement, crawlspace, attic, garage — and run a hand slowly around joints, seams and boots. Supply leaks are easy to feel. For return leaks, hold a strip of tissue near the seams; it will pull toward the duct rather than away from it. Look for daylight at register boots, disconnected flex duct lying in insulation, and sections held together by nothing but tape that gave up a decade ago.
Finding a visibly disconnected duct is common and satisfying. A branch that fell off its takeoff is sending 100% of that room's air into the attic, and homeowners usually find one by crawling up there to look.
What sealing actually involves
Not cloth duct tape. Despite the name, standard duct tape dries out and releases, especially at the temperature extremes ducts see. Real duct sealing uses:
- Mastic — a thick paste brushed over joints that stays flexible and seals permanently. The standard for accessible ductwork.
- Foil tape rated for the purpose (UL 181), used on clean metal surfaces, usually with mastic over it on anything important.
- Mechanical fasteners first — sealant is not a substitute for ducts that are properly screwed together and supported.
- Aerosol sealing for everything you cannot reach, which is most of the system in a finished house. Aeroseal pressurizes the duct system and injects a sealant aerosol that collects at the leak edges and closes them from inside — we cover how that works in this piece.
Sealing is not duct cleaning
These get conflated constantly. Duct cleaning removes accumulated debris from inside the ducts — worth doing after renovation work, after rodent activity, or when there is visible contamination. Duct sealing closes the gaps air escapes through. Cleaning does not improve efficiency; sealing does. A company that answers an efficiency complaint by selling a cleaning has answered a different question.
Do this before you replace equipment
If you are weighing a new furnace or air conditioner because the house is uncomfortable, get the duct system assessed first. There are two good reasons. The obvious one is that sealing may solve the problem for a fraction of the cost. The less obvious one: new equipment should be sized to a Manual J load calculation, and a load calculation performed on a house with a leaky, unbalanced duct system will size equipment to compensate for a defect you are about to keep paying for.
Smith assesses duct condition and airflow as part of heating and AC maintenance, and handles sealing including Aeroseal. If the ducts turn out to be fine, we will tell you that too.
Frequently asked questions
How do I know if my air ducts are leaking?
Set the thermostat fan to ON and feel around accessible duct joints, seams and register boots for escaping air; for return leaks, hold a tissue near the seams and watch it pull toward the duct. Weak airflow in distant rooms, rapid dust return after cleaning, and an attic that feels conditioned near the ducts are all indicators.
Is duct sealing worth it?
In homes with ductwork running through an attic, crawlspace or garage, usually yes. Air lost in those spaces never reaches a room, so it is a complete waste of the energy used to condition it. Sealing commonly resolves the comfort complaint that would otherwise prompt an equipment replacement costing considerably more.
Is duct cleaning the same as duct sealing?
No. Duct cleaning removes debris from inside the ducts and is worth doing after renovation, rodent activity or visible contamination, but it does not improve efficiency. Duct sealing closes the gaps that conditioned air escapes through, which is what affects comfort, run time and energy use. They solve different problems.
Can I seal my own ducts with duct tape?
Cloth duct tape is the wrong material despite the name — it dries out and releases at the temperatures ductwork sees. Accessible joints can be sealed by a homeowner using duct mastic or UL 181-rated foil tape over mechanically fastened connections. Ducts inside finished walls and ceilings require aerosol sealing from the inside.