Air Leakage and Duct Leakage Testing Fundamentals
Why we measure leakage
A building envelope and its duct system both leak, and the two leaks have different consequences. Envelope leakage drives infiltration, which affects heating and cooling loads, moisture transport, and indoor air quality. Duct leakage moves conditioned air into unconditioned space (supply leaks) or pulls outside air into the system (return leaks), which changes both delivered capacity and pressure balance. Candidates should be able to explain the driving force: leakage flow is driven by a pressure difference, and flow rises roughly with the square root of that pressure difference for a fixed opening (orifice flow). That square-root relationship is why a test at 50 Pa is a standard reference point rather than a real-life condition.
Reference pressures and normalized metrics
Know the standard blower door reference of 50 Pa and the common outputs: CFM50, air changes per hour at 50 Pa (ACH50), and leakage area estimates such as effective leakage area. For ducts, the standard reference is 25 Pa across the duct boundary, expressed as CFM25, often normalized by conditioned floor area or by duct surface area. The trap: mixing reference pressures (reporting a duct result at 50 Pa, or comparing an envelope number to a duct number) invalidates the comparison. Another trap is treating a normalized metric as a physical measurement; ACH50 depends on volume, so two identical envelopes of different size give different ACH50.
Worked example
A 2,000 ft² house with 16,000 ft³ volume measures 3,200 CFM50. ACH50 = 3,200 × 60 ÷ 16,000 = 12.0 ACH50. If a duct test on the same house gives 120 CFM25, do not add it to the envelope figure; they are separate systems at separate pressures.
Rule of thumb
Flow scales with the square root of pressure: doubling pressure raises leakage about 40%, not 100%.