A duct leakage test measures how much air escapes from your heating and cooling ducts. A technician seals the registers, connects a calibrated fan such as a Duct Blaster to the system, and pushes the ducts to a standard pressure of 25 pascals. The airflow needed to hold that pressure is the leakage, reported in cubic feet per minute at 25 pascals (CFM25). A second version, using a blower door, isolates only the leakage to outside the house. Together they tell you whether sealing is worth doing and whether it worked.
What a duct leakage test actually measures
Think of the duct system as a balloon with holes. If you blow into it and hold it at a fixed pressure, the air you keep adding equals the air leaking out. A duct test works the same way.
The U.S. Department of Energy's Building America Solution Center explains the method in its duct leakage tests guide, which we read on September 27, 2026. The test is run at 25 pascals, or 0.10 inch of water column, and the result is written as CFM25. Because every tester uses the same pressure, results from different houses and different visits can be compared.
So a result of, say, 200 CFM25 means the fan had to supply 200 cubic feet of air per minute to keep the ducts at that pressure. That number is air your system was pushing somewhere other than the registers.
A duct test does not tell you where the leaks are. It gives one number for the whole system. Finding the holes is a separate job with smoke, a pressure pan or your eyes.
Total leakage vs leakage to outside
There are two tests, and they answer different questions.
Total duct leakage measures air lost from all the ductwork connected to the system, inside or outside the home's thermal envelope. It is the simpler test: seal the registers, pressurize the ducts, read the fan.
Leakage to outside measures only the air lost from the ducts to places outside the conditioned space, such as a vented attic, a vented crawl space or a garage. To isolate it, the tester runs a blower door at the same time and pressurizes the house to match the ducts. With the house and ducts at the same pressure, leaks from ducts into the living space stop counting, and only leaks to the outside register on the duct fan.
Leakage to outside matters most for your energy use. Air that leaks from a duct inside a heated room still heats that room. Air that leaks into the attic is gone. If your ducts run through an attic or crawl space, ask for both tests.
How the test is run, step by step
Per the Building America Solution Center, the basic procedure is:
- Seal all supply registers and return grilles with cardboard, covers or adhesive plastic.
- Connect the duct tester to the largest return or to the air handler.
- For leakage to outside, set up a blower door and pressurize the house to match the duct test pressure.
- Bring the ducts to 25 pascals and record the airflow.
The furnace or air handler stays off throughout. In a house with gas appliances, the tester should also account for any water heater or furnace sharing the space, since running a blower door changes house pressure. A qualified tester handles that as part of the setup.
The Solution Center names a certified HERS rater, credentialed through RESNET, as the person who performs and documents the test. Some home energy auditors and HVAC contractors carry the equipment as well. Ask which test they will run and whether the report will state CFM25 for each.
Reading the number: compare it to your house, not to a stranger's
A raw CFM25 figure means little on its own. A big house with long duct runs will leak more air than a small one with the same workmanship. That is why codes and programs express limits per 100 square feet of conditioned floor area.
The Building America Solution Center lists these limits for new homes:
- 2012 to 2021 IECC model code: total duct leakage of 4 CFM25 or less per 100 square feet of conditioned floor area. The 2021 edition added a limit of 8 CFM25 per 100 square feet for ducts entirely inside the thermal envelope.
- ENERGY STAR single-family new homes: at rough-in, 4 CFM25 per 100 square feet or 40 CFM25; at final, 8 CFM25 per 100 square feet or 80 CFM25.
These are targets for new construction. They are not pass or fail lines for a 30-year-old house, and no code requires your existing ducts to meet them unless your jurisdiction says so for a specific permit. Adoption varies by state and city, and your local building department governs.
What they do give you is a frame. To see where your house sits, divide your CFM25 by your conditioned floor area in hundreds of square feet. A house far above the new-home level has a lot to gain from sealing.
What the test cannot tell you
We are not going to put a number on what sealing will save you, or on what a test costs. No public source gives a savings figure for your ducts, and a price depends on the region, the size of the house, whether a blower door is also used, and whether testing is bundled with an energy audit. We state the variables and refuse the number.
The test also cannot tell you:
- Where the leaks are. It is one number for the whole system.
- Whether the ducts are the right size or deliver the right airflow to each room. That is duct design, a separate job.
- How much heat the ducts lose through their walls. A tight but bare duct in a hot attic still gains heat. That is an insulation question.
For the house itself, a blower door test measures whole-house air leakage. Our explainer on what a blower door number means covers that test, which often gets run on the same visit.
Before and after: using the test to prove the sealing worked
The most useful way to run a duct leakage test is twice: once before sealing and once after.
The before number tells you whether the ducts are worth the effort and gives a baseline. The after number tells you whether the sealing did anything. Without it, you are trusting that every joint got covered, including the ones behind the furnace and at the far end of the attic.
ENERGY STAR's duct sealing guide, read on September 27, 2026, says that in a typical house about 20 to 30 percent of the air moving through the duct system is lost to leaks, holes and poorly connected ducts. A test tells you whether your house is typical.
Sealing reachable joints with mastic or a listed foil tape is homeowner work in many attics and basements. For ducts you cannot reach, some contractors use an aerosol sealant injected from inside the ducts, a method that grew out of research at Lawrence Berkeley National Laboratory. Those systems report their own before and after figures.
Can you test your ducts yourself?
Not in the calibrated sense. A duct tester is a specialized, calibrated fan with a digital pressure gauge, and running the leakage-to-outside version also needs a blower door. Buying the equipment for one house rarely makes sense.
What you can do yourself is find leaks. A smoke pencil or incense stick held at joints while the blower runs shows air movement. Our guide to finding air leaks with a smoke pencil covers the method, and our DIY home energy audit puts ducts into the full walk-through of the house.
A DIY check tells you where. A duct leakage test tells you how much. You want both.
FAQ
What is a good duct leakage test result?
For new homes, the 2012 to 2021 IECC model code sets total leakage at 4 CFM25 or less per 100 square feet of conditioned floor area, per DOE's Building America Solution Center. Existing homes are not held to that number, but it is a useful benchmark.
What is the difference between total leakage and leakage to outside?
Total leakage counts air lost anywhere from the ducts. Leakage to outside counts only air lost to unconditioned spaces like a vented attic or crawl space, and it needs a blower door running at the same time.
Who does a duct leakage test?
The Building America Solution Center names a RESNET-certified HERS rater. Some energy auditors and HVAC contractors also own the equipment. Ask for CFM25 figures in writing.
Should I test before or after sealing ducts?
Both. The before test sets a baseline and shows whether sealing is worthwhile. The after test shows whether it worked.
