📊 Leakage as % of System Airflow
📏 CFM25 Per 100 ft²
Compliance Check
Full Calculation Table
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How to Use the Duct Blaster Calculator
Choose Total Leakage if the test measured all duct leakage, or Leakage to Outside if the test setup isolated leaks that go to unconditioned space only.
Enter the airflow shown on the duct blaster fan at 25 pascals, and the total design or measured system airflow so the calculator can express leakage as a percentage.
Enter conditioned floor area. ENERGY STAR limits are set per 100 square feet of floor area, and the calculator also reports your result as CFM25 per 100 square feet.
Pick ENERGY STAR rough-in or final limits, or enter a custom percentage of system airflow, then click Calculate Leakage to see a pass or fail result along with the leakage share of delivered airflow.
Understanding Duct Blaster Testing
A duct blaster test measures how much air escapes a duct system before it ever reaches a room. In many Canadian homes, ducts run through attics, crawlspaces, and unheated basements where a poorly sealed joint or a disconnected boot can quietly waste a large share of the heating and cooling energy the furnace or heat pump produces. The test isolates that waste with a number you can act on.
How the Test Works
A calibrated fan, similar in concept to a blower door but sized for ductwork, connects to the return or a sealed register. All other supply and return openings are temporarily sealed with tape or test plugs. The fan pressurizes the duct system to a standard test pressure, almost always 25 pascals, and the airflow required to hold that pressure is the CFM25 reading. More leakage means the fan has to push more air to maintain the same pressure. For a design-stage figure to compare the result against, the duct leakage calculator in duct design estimates leakage before anything is built.
Total Leakage vs. Leakage to Outside
Total leakage counts every bit of air escaping the duct system, including leaks into conditioned space such as a supply boot leaking into a wall cavity that still ends up inside the house. Leakage to outside counts only air escaping to unconditioned space, like an attic or exterior wall cavity, which is the leakage that actually leaves the building envelope and drives up energy bills. A second test setup, often using a blower door in combination with the duct blaster, isolates the leakage-to-outside figure specifically.
Reading the Result Against Duct Leakage Targets
Building codes often prescribe how ducts are sealed rather than setting a leakage rate, so a numeric pass mark usually comes from an energy program or the project contract. The most widely used numeric benchmark is ENERGY STAR Single-Family New Homes in the United States. As summarized by the U.S. Department of Energy's Building America Solution Center, it limits total leakage to the greater of 4 CFM25 per 100 square feet of floor area or 40 CFM25 at rough-in, or the greater of 8 per 100 square feet or 80 CFM25 at final, and limits leakage to outdoors to the greater of 4 per 100 square feet or 40 CFM25. Those are the preset targets in this calculator. For a program that sets its limit as a percentage of system airflow, use the custom target, and always confirm the specific number your project needs to hit before treating a result as pass or fail.
What a High Leakage Result Means for Energy Use
Older, unsealed systems can leak a large share of the air they move, and the part that escapes to an attic or crawlspace is heating or cooling energy that never reaches a room. The duct sealing savings calculator estimates what sealing would be worth. Beyond wasted energy, high leakage on the return side can pull in dusty attic or crawlspace air and depressurize the mechanical room enough to affect combustion appliance safety, which is why duct sealing is often paired with a combustion safety check during a full retrofit.
After the Test: Sealing and Verification
If a system fails its target, mastic and mesh at accessible joints, plus sealing boots and plenum connections, usually brings leakage down significantly. Ontario's 2024 Building Code, for instance, requires joints on ducts in unconditioned space to be sealed to SMACNA Class A and supply ducts in conditioned space to Class C. A second duct blaster test after sealing confirms the improvement. Pair this tool with the airflow measurement calculator to confirm registers are still delivering design airflow once the ducts are sealed.
Frequently Asked Questions
A duct blaster test uses a calibrated fan connected to the duct system to pressurize it to a standard test pressure, usually 25 pascals, while all supply and return registers are sealed. The airflow needed to maintain that pressure quantifies how much air is leaking out of the ducts. The result, called CFM25, feeds directly into this calculator to produce a percentage and a pass or fail result.
Total leakage measures all air escaping the duct system, including leaks into conditioned space. Leakage to outside measures only the air escaping to unconditioned space, such as an attic or crawlspace, which is the leakage that actually wastes energy and affects the blower door test result. Programs often set separate, tighter limits for leakage to outside since it has the biggest energy impact.
Building codes such as Ontario's prescribe how duct joints are sealed rather than setting a leakage rate, so any numeric target usually comes from an energy program or the project contract. A widely used benchmark is ENERGY STAR Single-Family New Homes in the United States: for total leakage, the greater of 4 CFM25 per 100 square feet of floor area or 40 CFM25 at rough-in, or the greater of 8 per 100 square feet or 80 CFM25 at final, and for leakage to outdoors the greater of 4 per 100 square feet or 40 CFM25. Always confirm the specific target that applies to your project with the program administrator or authority having jurisdiction.
Not accurately. A calibrated duct blaster fan with a manometer is required to get a repeatable CFM25 reading that codes and energy programs will accept. Visual inspection and a smoke test can help locate individual leaks, but they cannot quantify total system leakage the way a proper duct blaster test can.
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