Why duct leaks matter so much
In many homes, a meaningful share of the air a heating and cooling system moves never reaches the rooms it was meant for. It escapes through gaps at joints, disconnected boots, unsealed plenums and torn flex duct, most of which sit in unconditioned spaces. You pay to heat or cool that air, and it disappears into the attic or crawlspace.
Leaks on the return side can be even more troublesome. Return ducts operate under negative pressure, so any gap pulls air in from wherever the duct runs. In an attic, that means hot, dusty air in summer. In a crawlspace, it can mean damp, musty air. In a garage, it can include exhaust fumes. That air then passes through the system and into your living space.
Duct leaks also create pressure imbalances in the home. When supply ducts leak into the attic, the house can become depressurized, drawing in outdoor air through gaps around windows, outlets and doors. When returns leak, the reverse can occur. These imbalances affect comfort and, in homes with fuel-burning appliances, can interfere with proper venting.
Because leaks are usually hidden, the best starting point is measurement. Testing shows how much the ducts leak and whether sealing is likely to deliver a meaningful improvement. If the ducts are too damaged or undersized to be worth sealing, the alternative is outlined on our Ductwork Install And Replace page.
Clues that your ducts may be leaking
None of these is proof on its own, but together they make a strong case for testing.
- High bills with newer equipmentIf you installed an efficient system and bills barely changed, leaking ducts may be wasting much of the improvement.
- Dusty house soon after cleaningReturn leaks in dusty spaces bypass the filter and deliver dust straight to rooms.
- Rooms far from the equipment underperformLeaks along the way mean less air arrives at the end of long duct runs.
- Hot upstairs rooms in summerSupply ducts leaking in a hot attic lose cooling before it reaches the rooms below.
- Visible gaps or loose tapeOld cloth-backed duct tape dries out and falls away, leaving joints open.
- Musty odors when the system runsReturn ducts pulling air from a damp crawlspace or basement can carry those odors through the house.
Paying to heat the attic?
Call to schedule leakage testing. You will see how much your ducts leak and what sealing is likely to accomplish.
How duct sealing is carried out
Most sealing projects take a day, including testing before and after.
Baseline leakage test
A calibrated fan pressurizes the duct system while registers are sealed off, measuring how much air escapes at a standard test pressure.
Visual inspection
We inspect accessible ducts in attics, basements and crawlspaces, looking for disconnected sections, open plenums and torn flex duct.
Manual sealing
Major leaks at joints, collars, plenums and boots are sealed with mastic and fiberglass mesh or approved foil tape, and disconnected sections are reattached and supported.
Aerosol sealing where suitable
For inaccessible ducts, an aerosol sealant can be blown through the pressurized system, depositing particles that seal small leaks from the inside.
Final leakage test
The test is repeated so you can see the measured reduction in leakage, and the results are recorded in writing.
What influences duct sealing results and cost
Accessibility determines how much sealing can be done manually. Ducts in an open basement are straightforward. Ducts buried in insulation, running through tight attics or enclosed in walls may call for aerosol sealing. Some duct systems are in such poor condition that sealing is not worthwhile and replacement is the better investment.
The size of the duct system, the number of registers and the extent of damage all affect labor. Testing before sealing establishes what can be achieved, and testing after confirms the result. We do not quote improvements we cannot measure.
Mastic versus tape
Duct mastic is a thick paste that is brushed onto joints and seams, often reinforced with fiberglass mesh on larger gaps. Once cured, it forms a durable, flexible seal that holds up to temperature swings for decades. It is messier to apply than tape but lasts far longer.
Standard cloth duct tape is not suitable for sealing ducts because its adhesive fails with heat and time. Foil-backed tapes rated for duct sealing perform much better and are appropriate in some locations. We use mastic for most joints and reserve approved tapes for places where mastic is impractical.
Sealing and air quality
Sealing return leaks means the system draws air only from the rooms it serves, through the filter. That reduces dust and can cut down on odors and humidity carried in from attics, crawlspaces and garages. For households dealing with allergies or dust complaints, return sealing often delivers visible results.
If air quality is your primary concern, testing through our Indoor Air Quality Testing service can confirm whether duct leakage is contributing, and sealing can be combined with filtration upgrades for greater effect.
Where the biggest leaks usually hide
Some of the largest leaks are found right at the equipment. The connection between the furnace or air handler and the supply and return plenums is often poorly sealed, and because the blower creates its strongest pressure at that point, gaps there waste a disproportionate amount of air. Filter slots without covers are another frequent culprit, pulling unfiltered air into the return side every time the blower runs.
Register boots, where the duct meets the floor, wall or ceiling, are the next common source. The gap between the boot and the drywall or subfloor lets air escape into wall and floor cavities. Sealing these from inside the room is often straightforward. Panned joist returns, where the space between floor joists is used as a return duct, can leak significantly and are worth inspecting in older homes.
Duct sealing questions
It varies widely depending on the installation and age of the system. Older homes with ducts in attics and crawlspaces often leak a substantial share of their airflow. Testing is the only way to know how your system performs.
Aerosol sealants used in residential duct systems are designed for that purpose. During the process registers are covered and the system is isolated, and the sealant is allowed to set before normal operation resumes. We explain the product and process before starting.
Homeowners can seal accessible joints with mastic, particularly in basements. The benefit of professional sealing is testing before and after, access to hidden sections and aerosol options for ducts that cannot be reached.
Sealing should not cause problems in a properly sized system. In some cases, sealing increases airflow to rooms and slightly raises static pressure. We check static pressure after sealing to confirm the system remains within its operating range.
It can. Whistling at gaps and rushing sounds from leaks are often reduced once joints are sealed. Noise from undersized ducts or high static pressure is a separate issue that sealing alone may not solve.
Pages related to duct sealing
Duct condition, air quality and equipment efficiency are closely linked.