Quick answer
Duct leakage is the air that escapes from, or is pulled into, a heating and cooling duct system through gaps, holes and loose joints instead of moving between the equipment and the rooms.
It is measured with a duct leakage test at 25 pascals and reported as cfm25 per 100 square feet of floor area. ENERGY STAR estimates typical systems lose about 20 to 30 percent of the air they move.
Quick facts
The key facts about Duct Leakage, with sources:
- Typical loss
- About 20%–30% of air moved 2
- Test pressure
- 25 Pa (0.1 in. w.c.) 4
- Test standards
- ANSI/RESNET/ICC 380 or ASTM E1554 4
- 2021 IECC limit (as adopted in Sanger, TX)
- ≤4 cfm25 per 100 sq ft; ≤3 with air handler not installed 3
- 2024 IECC alteration limit
- ≤12.0 cfm25 per 100 sq ft when ≥25% of ducts or registers move or length grows ≥25% 4
Key takeaways
- Leaky ducts waste air you paid to heat or cool; ENERGY STAR puts typical loss at 20% to 30% 2.
- Supply leaks push air out; return leaks pull attic, crawl space or garage air in.
- Leakage is tested at 25 Pa and expressed as cfm25 per 100 sq ft of floor area 4.
- The 2021 IECC limit for new systems is 4 cfm25 per 100 sq ft 3; the 2024 IECC adds a 12.0 limit for major alterations 4.
- Seal with mastic or metal tape; never plain duct tape 2.
What duct leakage is
A duct system should be a closed loop: air leaves the blower, reaches the rooms, and returns. Every gap breaks that loop. Joints between duct sections, the seams of the plenum, the boots behind registers and the connection to the equipment are the usual leak points.
Supply-side leaks blow heated or cooled air into the attic, crawl space or wall. Return-side leaks suck in air from those spaces: hot attic air in summer, damp crawl space air, or garage air with fumes. Both waste energy. Return leaks can also hurt indoor air and, near combustion appliances, change how they vent.
Leaks inside the conditioned space cost less, because the air stays in the house. That is why codes treat ducts fully inside the home differently 3.
How a duct leakage test works
This is the total-leakage method the IECC describes 4. A trained tester runs it.
- Seal every supply register and return grille with tape or film. If registers are not yet installed, seal the boot openings.
- Connect a calibrated fan (a duct tester) to the system, usually at the return or the air handler.
- Run the fan to hold the ducts at 25 pascals (0.1 inch water column) above the house.
- Read the airflow needed to hold that pressure. That is the leakage, in cfm25.
- Divide by conditioned floor area and multiply by 100 to get cfm25 per 100 sq ft.
- The tester signs a written report for the code official.
A second test, leakage to outside, also pressurizes the house with a blower door so that only leaks to the outdoors are counted. It needs both tools.
Where leaks hide, from equipment to room
- Air handler or furnace cabinet seams and filter slot
- → supply and return plenum connections
- → trunk seams and takeoffs
- → branch joints and flex duct collars
- → register boots where they meet drywall
- → return platforms and panned joist returns
Kinds of duct leakage measurements
| Measurement | What it counts | Used for |
|---|---|---|
| Total leakage | All leaks, inside or outside the house, at 25 Pa | IECC compliance 4 |
| Leakage to outside | Only leaks to the outdoors or unconditioned space | Energy programs; better link to energy loss |
| Rough-in test | Before drywall, with or without the air handler | Catch leaks while easy to fix 3 |
| Post-construction test | After the house is finished | Final compliance 3 |
Worked example: IECC limits for a 2,000 sq ft house
Limits from the 2021 IECC as adopted in Sanger, Texas 3, and the 2024 IECC text reproduced in an ICC code change document 4. The test result of 180 cfm25 is assumed.
| Check | Math | Allowed (cfm25) |
|---|---|---|
| 2021 IECC, rough-in or final, with air handler | 2,000 ÷ 100 × 4.0 | 80 |
| 2021 IECC, rough-in, air handler not installed | 2,000 ÷ 100 × 3.0 | 60 |
| 2024 IECC, rough-in, equipment not installed | 2,000 ÷ 100 × 3.0 | 60 |
| 2024 IECC, alteration moving ≥25% of ducts | 2,000 ÷ 100 × 12.0 | 240 |
| Measured (assumed) | 180 cfm25 ÷ 20 | 9.0 cfm25 per 100 sq ft |
At 9.0 per 100 sq ft, this system would fail a new-construction test by a wide margin but pass the 2024 alteration limit. Sealing the plenum connections and boots usually brings the biggest drop. Note that cfm25 is leakage at test pressure, not the share of blower airflow lost in operation.
Where duct leakage matters most
- Attic ducts in hot climates: supply leaks dump cooling; return leaks pull in hot air.
- Crawl spaces: return leaks can pull in damp, musty air.
- Garages: return leaks can pull in fumes; code requires sealed, unopened ducts through the garage wall 5.
- Near gas appliances: strong return leaks can lower pressure in a furnace room.
- On a new-home energy report or permit file, as the test result.
What sealing ducts gets you
What it does well
- More of the air you pay for reaches the rooms.
- Better comfort in rooms at the end of runs.
- Less dust and attic air pulled into the house.
- Often lets a smaller unit do the job.
Where it falls short
- Hard-to-reach ducts behind walls may stay leaky.
- Sealing without testing leaves the result unknown.
- Very leaky old systems may be cheaper to replace than seal.
What a leakage test does not show
Total leakage counts every leak, including harmless ones inside the house. Leakage to outside is a closer guide to energy loss but takes more equipment.
The cfm25 number is measured at a fixed test pressure. Real operating pressures vary across the system, so a leak near the blower loses more air than one at a far register. And a tight duct system can still be undersized; check static pressure too.
Cost drivers
What moves the price: how much duct is reachable, attic and crawl access, the number of boots and joints, whether sealing is by hand with mastic or by an aerosol process from inside the ducts, and whether insulation must be removed and replaced.
Testing before and after is the only way to know what you got. HyreHVAC does not print sealing prices without enough local quotes; see ductwork services and the repair cost calculator.
Checklist: a duct sealing job done right
- Test first and record cfm25 (and leakage to outside if possible).
- Seal the biggest leaks first: equipment cabinet, plenums, return platform.
- Seal joints, seams and fittings with mastic or another approved method 5.
- Seal boots to the drywall or floor.
- Repair or replace crushed, torn or disconnected flex duct.
- Re-insulate any duct uncovered outside conditioned space.
- Test again and keep the signed report 4.
Keeping ducts tight
- Look at accessible ducts each year for loose joints or torn wrap.
- Keep stored items off attic and crawl space ducts.
- Have pest damage repaired and re-sealed.
- Re-test after any duct changes.
Warning signs of leaky ducts
- High summer and winter bills.
- Rooms that are hard to heat or cool, or stuffy.
- Ducts in the attic, crawl space or garage.
- Tangled or kinked flex duct.
- Dusty air or musty smells when the blower runs.
- If a gas appliance shares the space and returns leak badly, ask a pro to test for backdrafting.
Sources: [2]
IECC duct leakage rules by edition
2021 IECC duct testing (cited as section 403.3.4 on the Sanger form). As adopted in Sanger, Texas: total leakage at 25 Pa no more than 4 cfm per 100 sq ft of conditioned floor area for both rough-in and post-construction tests, or 3 cfm per 100 sq ft at rough-in if the air handler is not installed.
No test needed if the air handler and all ducts are inside conditioned space 3.
2024 IECC (R403.3.7 duct system testing). Each duct system is tested per ANSI/RESNET/ICC 380 or ASTM E1554 at 25 Pa, supply and return together, with a signed written report.
Exceptions cover ventilation-only ducts and systems with no more than 10 feet of duct outside the equipment, fully in conditioned space and with no building-cavity or gypsum plenums.
Rough-in testing without equipment allows 3.0 cfm per 100 sq ft. Maximum leakage for other cases is set in Table R403.3.8.
Under R503.1.2.3, an alteration that relocates 25% or more of registers or ductwork, or lengthens the system by 25% or more, must test at 12.0 cfm per 100 sq ft or less, unless the ducts are fully inside conditioned space 4.
Adoption. States and cities adopt different editions and amend them. Ask your building department which applies.
Duct leakage vs related terms
| Term | What it measures | Relation |
|---|---|---|
| Duct leakage (cfm25) | Air lost from ducts at 25 Pa | Duct tightness |
| ACH50 | House leakage at 50 Pa | House tightness, measured with a blower door |
| Duct insulation | Heat loss through duct walls | A separate loss from leaks |
| Ductwork | The duct system | What is being tested |
| Energy audit | Whole-home assessment | Often includes a duct test |
Common misconceptions
- Myth Duct tape is for ducts.
- Reality ENERGY STAR says never use duct tape to seal ducts; it does not last 2.
- Myth Leaks inside walls do not matter.
- Reality Walls often connect to attics or outside; those leaks still cost energy.
- Myth New homes have tight ducts.
- Reality Only a test proves it; that is why the IECC requires one.
- Myth Duct cleaning fixes leaks.
- Reality Cleaning removes dust; it does not seal gaps. EPA recommends cleaning only as needed, not routinely 1.
Where duct leaks cost the most
Attic ducts in hot-summer states such as Texas, Arizona and Florida lose the most to leaks, because the attic can be far hotter than the house.
Local codes differ on how they enforce testing; Sanger, Texas, for example, publishes its own 2021 IECC test form with the limits above 3.
Cold-climate homes in Ohio more often have basement ducts, where leaks cost less but still matter.
When this matters to you
Get a duct leakage test when you replace equipment, buy a home with attic ducts, or have rooms that never feel right. If the ducts leak badly, sealing them before buying a new unit can let you buy a smaller one.
Next step: book an HVAC inspection that includes a duct test, or check how a home energy audit bundles one.
Questions about Duct Leakage
How much air do leaky ducts lose?
ENERGY STAR estimates about 20 to 30 percent of the air moving through a typical duct system is lost through leaks, holes and poor connections. A duct leakage test measures your actual figure.
What is a passing duct leakage test?
Under the 2021 IECC, as adopted in places like Sanger, Texas, new systems must leak no more than 4 cfm per 100 square feet of floor area at 25 pascals, or 3 cfm with the air handler not installed. Local codes may differ.
What does cfm25 mean?
It is the airflow, in cubic feet per minute, that leaks from the duct system when it is held at 25 pascals of pressure during the test. Dividing by floor area and multiplying by 100 gives cfm25 per 100 square feet.
How is a duct leakage test done?
The tester seals all registers and grilles, connects a calibrated fan to the ducts, and pressurizes them to 25 pascals. The fan airflow needed to hold that pressure is the leakage. The test follows ANSI/RESNET/ICC 380 or ASTM E1554.
Does the code require a duct test on an existing home?
Usually not for a simple equipment swap. The 2024 IECC requires a test at 12.0 cfm per 100 square feet or less when an alteration moves or adds 25 percent or more of the ducts or registers, unless the ducts are fully indoors. Check local adoption.
What is the best way to seal ducts?
Mastic sealant or metal foil tape on joints, seams and boots, as ENERGY STAR recommends. Plain cloth duct tape fails over time. Aerosol sealing from inside the ducts can reach hidden leaks.
What is the difference between total leakage and leakage to outside?
Total leakage counts every leak. Leakage to outside counts only leaks to the outdoors or unconditioned space, which waste energy. Leakage to outside needs a blower door running at the same time as the duct tester.
Can leaky ducts affect air quality?
Yes. Return-side leaks can pull in dust, insulation fibers, crawl space moisture or garage fumes. Near gas appliances, strong return leaks can affect venting, which is a reason to have a professional test.
How long does a duct leakage test take?
A total leakage test on a typical house usually takes an hour or two, most of it sealing and unsealing registers. Adding leakage to outside takes longer because a blower door is also set up.
Sources
- U.S. EPA, Should You Have the Air Ducts in Your Home Cleaned?, retrieved .
- ENERGY STAR, Duct Sealing, retrieved .
- City of Sanger, Texas, 2021 IECC Residential Energy Code Test Form, retrieved .
- ICC, code change proposal REC2D-10-23 (reproduces 2024 IECC R402.5.1.3, R403.3.7 and R503.1.2.3), retrieved .
- Town of Cave Creek, Arizona, 2021 I-Code Residential Inspection Checklist: Mechanical, retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: 2021 IECC limits copied from Sanger, Texas's official 2021 IECC test form.
2024 IECC test method, exceptions and alteration limit copied from the 2024 text reproduced in ICC document REC2D-10-23 (the Table R403.3.8 values themselves were not reproduced there and are not quoted); loss estimate from ENERGY STAR, read 10 October 2026.
The 180 cfm25 result is assumed.
Suggest a correction. We fix errors and say what changed.