Blower Door and Duct Leakage Testing in a Tight Multifamily Unit
An assessor tests a 900 ft² apartment on the third floor of a four-story building. A blower door with a single 4-inch ring is set up; the gauge reports 3,150 cfm50, which is 3.5 air changes per hour at 50 Pa. The building corridor is pressurized and the unit door has a 1-inch undercut. A duct leakage test to outside reports 210 cfm25, about 19% of the air handler's 1,100 cfm rated flow. The owner complains of stuffiness and high winter humidity. The assessor notices the gauge zero drifted between tests and that the manometer was referenced to the corridor, not the outdoors.
1. Evaluate the validity of the 3,150 cfm50 result and describe corrections.
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The result is not trustworthy as reported. A blower door measures pressure difference between the conditioned zone and a reference; referencing to a pressurized corridor rather than outdoors biases the reading because the corridor pressure is not the true outdoor baseline. The 1-inch door undercut also creates a leakage path to the corridor that is included in the measurement, so the unit's envelope leakage is overstated. The gauge zero drift adds systematic error to every pressure reading and must be re-zeroed before and after each test.
Corrections: reference the manometer to outdoors with a tap run outside or through a window, seal or account for the corridor door undercut, re-zero the gauge, and repeat the test. Report the range from repeated tests rather than a single number. Also verify the ring configuration and flow range are appropriate; a single 4-inch ring may be near its measurement limits at 3,150 cfm50, so confirm the gauge is within its calibrated range and consider a larger ring.
2. Interpret the 210 cfm25 duct leakage in context and explain how it relates to the stuffiness and humidity complaints.
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210 cfm25 is 19% of rated airflow, which is high for a tight apartment and suggests leakage to unconditioned space such as the corridor, attic, or wall cavities. Leakage to outside at this rate both depressurizes or pressurizes the unit relative to the corridor and drives outdoor or corridor air into the living space, which can explain stuffiness if the corridor air is stale or if supply leakage reduces delivered airflow.
The humidity complaint is more nuanced: in winter, high indoor humidity usually points to moisture sources and poor ventilation rather than duct leakage alone, but return leakage from a humid corridor or from unconditioned space can import moisture. The assessor should determine whether leakage is on the supply or return side, since return leakage draws contaminants in while supply leakage pushes conditioned air out. Recommend sealing ducts at the air handler and plenums, verifying the ventilation strategy, and re-testing to a target below 10% of rated flow.