Duct Leakage Test Numbers Explained for Other Trades

Why this matters

A duct leakage certificate is now part of the paperwork on most new homes and many replacement jobs, and it gets read by people who never ran the test: the builder, the inspector, the insulator whose crew walked across the attic, the drywall sub whose boots got buried, the homeowner holding the sheet after a comfort complaint. Two numbers sit on most certificates and they answer two different questions. A trade that confuses them can concede a failure that was never theirs, or sign off on a system with one joint blowing straight into the garage ceiling because the total came in under the line.

The two tests, and the question each one asks

Both tests seal every supply register and return grille, connect a calibrated fan to the duct system, and push or pull air until the ducts sit at 25 pascals relative to a reference. The fan flow needed to hold that pressure is the leakage, written CFM25. What differs is the reference.

Total leakage references the ducts to the space around the test equipment, with the house at whatever pressure it happens to be at. It counts every leak: into the attic, the crawlspace, the garage, and into the wall cavities and rooms of the house itself. It answers the question "how tight is this duct system as built?"

Leakage to outside runs the same duct test while a blower door holds the house at the same pressure as the ducts. With the house and ducts at equal pressure, a leak from the duct into the house has no pressure across it and passes nothing. Only leaks to spaces outside the envelope still flow. It answers the question "how much air is this duct system throwing away or pulling in from unconditioned space?" That is the number tied to energy waste and to the house-pressure effects in Duct Leakage Outside the Envelope.

Because leakage to outside is a subset of total leakage, it should never read higher than total on the same system. If a certificate shows it higher, the test setup was off and the two numbers cannot be used together.

The normalization, and the code line

Both numbers are usually reported per 100 square feet of conditioned floor area so systems of different sizes can be compared. The IECC's prescriptive duct test, as adopted by your jurisdiction, holds total leakage to 4 CFM25 per 100 square feet in the 2012 through 2021 editions, with a tighter 3 CFM25 per 100 square feet required instead for a rough-in test run before the air handler is installed. Recent editions point to ANSI/RESNET/ICC 380 for the test procedure itself. The section number moved between editions (R403.2.2 in 2012, renumbered after), and the treatment of systems located entirely inside the thermal envelope changed between editions too, so read the one adopted where the house stands. The code line is on total leakage. Leakage to outside is the measure many efficiency programs and raters use; when a program governs the job, its number governs it.

A third unit you will meet: percent of system airflow. Some reports and older specifications state leakage as a share of the air handler's flow. On a nominal 2-ton system moving about 800 CFM, 86 CFM25 of total leakage would be written as 86 divided by 800, about 11 percent. That conversion divides a test-pressure leak by an operating flow, so it is a convenient label, not a measured fraction of the air lost in service. Before comparing a percent figure with a per-100-square-foot figure, convert one to the other and say which you started from.

A certificate, line by line

 DUCT LEAKAGE TEST - FINAL, air handler installed
 ------------------------------------------------
 Conditioned floor area     2,400 sq ft
 Test pressure              25 Pa, ducts WRT ref
 Registers and grilles      sealed
 ------------------------------------------------
 TOTAL LEAKAGE              86 CFM25
   per 100 sq ft = 86 / 24  = 3.58
   code line (total)          4.0   -> PASS
 LEAKAGE TO OUTSIDE         41 CFM25
   house held at 25 Pa by blower door
   per 100 sq ft = 41 / 24  = 1.71
 ------------------------------------------------
 Leaks inside envelope = 86 - 41 = 45 CFM25

Check the normalization from the other direction: 3.58 per 100 square feet over 24 hundreds is 86 CFM25, and 1.71 over 24 is 41. The last line is the one most readers skip: 45 of the 86 CFM25 leak into the house itself, where they cost little energy but can still misdeliver air between rooms.

Worked example: a pass that hides a problem

That system passed the code line at 3.58 against 4.0. The HVAC contractor is ready to close it. Then the homeowner of the room over the garage complains that the room smells like the car in winter.

What the certificate can and cannot show. CFM25 is a sum over every leak in the system. It says nothing about how the leakage is distributed. Say, as an illustration, that one panned return run through the garage ceiling joist bay was never sealed at its end and accounts for about 25 CFM25 on its own. That is 25 of the 41 CFM25 to outside, about 61 percent of the leakage to outside, through one joint. The remaining 16 CFM25 to outside are spread across dozens of minor seams.

Why that one joint matters more than its share. First, it is a return leak, so in operation it pulls garage air into the system instead of losing house air: car exhaust and fuel vapor go to every room. Second, 25 pascals is a test pressure, not an operating pressure. Return-side pressures near the air handler in operation are commonly two or three times that (a typical total external static pressure of 0.5 inches of water column is about 125 pascals across the whole system, supply and return together), so a joint close to the blower may pass more in service than its test share suggests. Third, the garage ceiling is part of the fire separation, and a return opening there is a code problem in its own right. See related: The Attached Garage as a Pressure Neighbor.

What should have caught it. The certificate passing is not evidence that no single leak matters. On any system with ducts or returns near a garage, a quick look with a smoke pencil at every joint in the garage ceiling, with the duct tester running, would have shown the smoke being sucked into the open pan end. Locating leaks is a method choice covered in Duct Blaster vs Smoke vs Pressure Pan Leak Location Method Decision Matrix.

What changes the answer. If that same 25 CFM25 joint had been a supply joint in a vented attic, it would have been an energy loss and a small house depressurizer, not an exhaust route. If the house had been tested only for total leakage, the problem would have been invisible on paper in both cases. And if the system had ducts entirely inside the envelope with no garage nearby, a 3.58 total would have been a good number with little hidden behind it.

Why rough-in and final read differently

A rough-in test is run after ducts are installed and before drywall, often before the air handler is set. A final test is run with everything in place. Expect them to differ, and know why.

• The air handler and its plenum connections are added between the two, and the cabinet, filter slot and coil case can be among the leakiest parts of the system. A rough-in without them reads lower, which is why the code holds that version to the tighter 3 rather than 4.
• Boot-to-drywall gaps do not exist at rough-in. The boots are sealed to a temporary cover. At final, the gap between the boot and the cut drywall opening is a leak path into the wall or ceiling cavity, and through the ceiling to the attic.
• Damage between tests. Insulators walk on flex, drywall crews knock boots loose, plumbers move a trunk to clear a drain. A final that reads well above the rough-in points at something done in between, and which trades worked in that space is usually the shortest list of suspects.
• Test setup changes. A rough-in run on depressurization and a final on pressurization, or with a different reference location, can disagree for reasons that have nothing to do with the ducts.

What the numbers do not tell you

Which trade caused a failure. A high final points at whatever changed since the last good reading. Without a rough-in on file, it points at the whole job.

Where the leaks are. CFM25 is one sum. Finding them takes smoke, a pressure pan or the tester itself with registers unsealed one by one.

What the system does in operation. The test pressure is a fixed 25 pascals across the system. In operation, pressures vary along the duct from high near the air handler to low at the registers, and the leaks close to the blower count for more.

Whether airflow to each room is right. A tight system can still deliver the wrong air to a room. Room-by-room airflow is a balancing job, not a leakage one.

Whether the house is safe. A pass on total leakage says nothing about a return leak in a combustion appliance zone or a garage. Those are found by looking, and a suspected combustion problem goes to a qualified gas technician. See related: The Combustion Appliance Zone and the Door That Closes It.

Reading a certificate with confidence

You have read the sheet right if you can say which test each number came from, what the floor area normalization was, which code or program line it is being held to and whether that line is on total or to outside, whether it was a rough-in or a final, and whether any duct or return runs near a garage or a combustion appliance. A pass on the number and a look at the joints in the risky spaces are two different checks, and a job needs both.

References

• International Energy Conservation Code duct leakage testing section (R403.2.2 in the 2012 edition, renumbered in later editions), as adopted by your jurisdiction
• RESNET/ICC 380, duct leakage test procedures used by raters and referenced in recent code editions
• See related: Duct Leakage Outside the Envelope; Duct Blaster vs Smoke vs Pressure Pan Leak Location Method Decision Matrix
• See related: Blower Door Numbers Explained for Other Trades; How to Find Duct Leaks Outside the Envelope