Ducts usually run through attics, crawl spaces and wall cavities you never see, so their leaks go unnoticed. Every leak on the supply side sends heated or cooled air into those spaces, and every leak on the return side pulls dusty, hot or cold air back in.
ENERGY STAR says that in a typical house, about 20 to 30% of the air moving through the ducts is lost to leaks, holes and poorly connected ducts, and the DOE puts duct air losses at about 30% of a cooling system's energy. A duct leakage test measures your system's leakage in a single short visit, and sealing the worst leaks is usually the fix.
Why duct leakage matters
- Wasted energy: the system runs longer to deliver the heat or cooling that leaks away. ENERGY STAR says leaky ducts can reduce heating and cooling efficiency by as much as 20%.
- Uneven rooms: rooms at the end of leaky runs get too little air.
- Indoor air: return leaks in an attic, crawl space or garage can pull dust, humidity and fumes into the system.
How a duct leakage test works
A technician seals every supply register and return grille, connects a calibrated fan to the system, and pressurizes the ducts to a standard 25 pascals. The airflow needed to hold that pressure is the leakage, reported in cubic feet per minute at 25 pascals (CFM25).
- Total leakage measures every leak, including those into conditioned space.
- Leakage to the outside is measured with the house also pressurized by a blower door, so only leaks to the attic, crawl space and outdoors count; it's the number that matters most for energy.
The test follows a standard procedure (ANSI/RESNET/ICC 380). It is quick to run, and a standalone test is relatively inexpensive.
Duct blaster vs. blower door
The two tests measure different things and are easy to confuse. A duct blaster, the calibrated fan described above, pressurizes only the duct system to 25 pascals and reports how much air the ducts leak.
A blower door pressurizes or depressurizes the whole house to measure the leakiness of the building shell, reported as air changes per hour. The two are run together for one duct number: with the blower door holding the house at the same pressure as the ducts, any air the duct blaster still has to move is leaking to the outside rather than into the house.
So a duct blaster alone gives total duct leakage, and a duct blaster plus a blower door gives leakage to the outside, the figure that drives your energy loss.
When a test is required
The International Energy Conservation Code requires duct leakage testing in new construction when ducts run outside the conditioned space, and many jurisdictions apply it to major replacements too. Utility programs and energy audits often include a test. Your local building department sets the requirement and the limit.
That limit is written as a maximum CFM25 per 100 square feet of conditioned floor area, so a larger house is allowed proportionally more leakage.
The 2021 IECC sets the numbers in Section R403.3.6, with the ducts held at 25 pascals. Total leakage must come in at or below these figures, in CFM25 per 100 square feet of conditioned floor area:
| When the duct system is tested | Maximum total leakage |
|---|---|
| Rough-in, air handler installed, and the final post-construction test | 4.0 CFM25 per 100 sq ft |
| Rough-in, before the air handler is installed | 3.0 CFM25 per 100 sq ft |
| All ducts and the air handler inside the thermal envelope | 8.0 CFM25 per 100 sq ft |
Work it out for your own house by multiplying the per-100 figure by your floor area. A 2,000 square foot home at the 4.0 limit may leak at most 80 CFM25, since 0.04 × 2,000 = 80. A house with every duct and the air handler inside the conditioned envelope gets the loosest 8.0 allowance, because any air that leaks there stays indoors.
Where ducts leak
- Connections between duct sections, and between ducts and the air handler.
- Boots, where ducts meet registers at the floor, wall or ceiling.
- Return plenums and panned joist returns, which are often barely sealed.
- Disconnected or crushed flex runs in attics and crawl spaces.
Finding leaks yourself
You can locate the biggest leaks without equipment before deciding on a paid test. Turn the system fan to "on" so air is moving, then work along the accessible ducts in the basement, crawl space or attic.
Run a hand slowly along joints, seams and the connections at the air handler. A supply leak feels like a jet of conditioned air, and a return leak like suction; holding a stick of incense or a smoke pencil near the same spots makes the moving air visible as the stream wavers or gets pulled in.
Look for the visual tells too: gray dust streaks fanning out from a joint, loose or hanging flex duct, and tape that has curled or fallen off. Mark each leak with a piece of tape so you can seal it or point it out later.
How to seal them
The right sealant depends on where the leak is and whether you can reach it. Reachable metal joints take tape or mastic, while ducts hidden inside walls need sealant blown through the system from the inside.
| Method | How it works | Durability | Best for |
|---|---|---|---|
| Mastic | A thick paste brushed over joints and seams that stays flexible as it cures | Long-lasting; fills irregular gaps | Any reachable joint, especially uneven ones |
| Foil tape listed to UL 181 | A metal-backed tape pressed onto clean, smooth surfaces | Holds where it bonds; ordinary cloth duct tape fails | Clean, smooth metal joints |
| Aerosol sealing | Sealant particles are blown through pressurized ducts and settle at the leaks from inside | Reaches leaks you cannot touch | Ducts hidden in walls and ceilings |
| Replacement | Damaged sections are removed and rebuilt | Permanent; also corrects crushed or wrongly sized runs | Crushed, disconnected or badly deteriorated ducts |
Seal boots to the drywall or floor with caulk or mastic, then insulate any ducts in unconditioned spaces once they are sealed. Aerosol sealing is done by specialized contractors for a significant fee, and full duct replacement is far more expensive, often a major project. See flexible vs. rigid ductwork for installation that prevents new leaks.
Is it worth it?
If your ducts run through an attic, crawl space or garage, and you have uneven rooms, high bills or dusty air, testing and sealing are usually worthwhile. A test before and after sealing shows how much it improved.
Put the loss figures in money terms. The DOE's roughly 30% duct-energy loss and ENERGY STAR's up-to-20% efficiency penalty mean that for every $100 you spend heating and cooling, somewhere around $20 to $30 pays for air that never reaches a room.
As an example, take a home spending $1,500 a year on heating and cooling. A 20% penalty is about $300 a year (1,500 × 0.20 = 300). Sealing the reachable leaks recovers much of that, and a before-and-after test shows how much your own system actually gained.
Frequently asked questions
How much air do leaky ducts lose?
About 20 to 30% of the air moving through the ducts in a typical house, according to ENERGY STAR.
How much does a duct leakage test cost?
A standalone test is relatively inexpensive; it is often bundled into a larger service or a code inspection.
Can I seal my own ducts?
Accessible joints, yes: use mastic or UL 181 foil tape, not cloth duct tape. Ducts inside walls need professional aerosol sealing or access work.
What is CFM25?
The airflow in cubic feet per minute that leaks from the ducts when they're held at 25 pascals of pressure, the standard test condition.
Will sealing ducts lower my electric bill?
Usually yes, if the ducts run through unconditioned space and leak noticeably. ENERGY STAR attributes up to a 20% heating and cooling efficiency loss to leaky ducts, and the DOE puts duct air loss near 30% of a cooling system's energy, so recovering part of that cuts runtime and the bill. A before-and-after test is the clearest way to confirm the gain on your own system.
How long does duct sealing last?
It depends on the material and the surface. Mastic brushed over joints stays flexible and holds up well, and foil tape listed to UL 181 lasts where it bonds to clean metal, while ordinary cloth duct tape dries out and peels. Sealant applied to clean, dry surfaces lasts far longer than a quick patch over dust or grease.
Do I need a duct test to pass inspection?
In new construction the International Energy Conservation Code requires a duct leakage test whenever ducts run outside the conditioned space, and many jurisdictions extend it to major replacements. Whether your specific job needs one is set by your local building department, which also sets the leakage limit. Utility rebates and energy-audit programs often require a passing test as well.
Sources & References
- ENERGY STAR: Duct sealing (20 to 30% of air lost in a typical house)
- ENERGY STAR: Duct sealing benefits (efficiency reduced by as much as 20%)
- U.S. DOE: Energy Saver 101, Home Cooling (PDF), duct losses about 30%
- U.S. DOE Building Science Education Solution Center: Duct Leakage Testing (2021 IECC R403.3.6 limits)