Building Envelope and Duct System Diagnostics
Reading pressure and flow patterns
Diagnostics go beyond a single number. Pressure mapping with a manometer reveals which zones are connected: a closed interior door with a large pressure difference across it indicates a blocked return path, while a small difference suggests transfer openings. Zonal pressure testing, where individual rooms or the garage are monitored during a blower door test, identifies pathways between conditioned and unconditioned space. Candidates should recognize that a garage at negative pressure relative to the house is a safety and air-quality concern because it can pull vehicle exhaust indoors.
Finding and interpreting leakage
Thermal imaging and smoke visualization locate leaks that the flow number only quantifies. The trap in questions is confusing a large total leakage number with a specific location, or assuming a high ACH50 always means a drafty house when the real issue may be a connected attic or crawlspace. Another common trap is misreading duct leakage: a supply leak in an unconditioned attic loses conditioned air and can pressurize the attic, while a return leak in the same attic draws hot or cold attic air into the system and depressurizes the attic.
Interaction with combustion appliances
Any diagnostic that depressurizes a house can backdraft naturally drafting appliances. Candidates must know to check for spillage and to avoid creating hazardous conditions during testing. A worst-case depressurization test is the standard check.
Worked example
During a blower door test, a bedroom shows −35 Pa relative to the main body while the house is at −50 Pa, meaning the bedroom is only 15 Pa below the house. That suggests a large transfer path. If instead the bedroom reads −50 Pa, the door is effectively sealed and the return path is inadequate.