Blower Door and Duct Leakage Measurement Procedures
Setting up a valid envelope test
A blower door test requires a calibrated fan, a manometer measuring both building pressure and fan pressure (or fan flow), and a building prepared in a defined state: exterior doors and windows closed, interior doors in a specified position, combustion appliances and exhaust fans off, and openings to unconditioned space (attic hatches, crawlspace doors) in their normal condition. The candidate must know that the test measures the building pressure relative to outdoors, typically with a reference tube run outside, and that fan flow is derived from the fan's calibration curve at the measured fan pressure.
Multi-point versus single-point
A proper test is multi-point: flows are recorded across a range of induced pressures (commonly about 15 to 60 Pa), then a curve is fitted and extrapolated to 50 Pa. A single-point test at 50 Pa is faster but more error-prone in windy conditions. The trap: a question describing a one-point test presented as a full diagnostic, or a test where building pressure never stabilized.
Duct testing procedure
Duct leakage is measured by pressurizing the duct system with a duct blaster (or a blower door adapted with a calibrated flow device) while the registers are taped and the air handler is sealed or isolated. The pressure reference is the pressure difference between the duct interior and the space surrounding the duct. Two configurations matter: testing with the house at the same pressure as outdoors isolates duct leakage from envelope leakage, while testing with the house depressurized measures total duct leakage including leakage to outdoors. Candidates should distinguish total duct leakage from leakage to outside.
Worked example
A duct test records 60 Pa duct pressure with 180 CFM of fan flow. If the target reference is 25 Pa, apply the square-root rule: CFM25 ≈ 180 × √(25/60) = 180 × 0.645 ≈ 116 CFM25. Report the corrected value and note the correction method.