Which term defines uncontrolled inward air leakage to conditioned spaces through unintentional openings, caused by the same pressure differences that induce exfiltration?

• A. Infiltration (correct)
• B. Ventilation
• C. IAQ
• D. Fenestration

Answer: (A)

Infiltration is the unintended, uncontrolled inward movement of outdoor air into a conditioned space through cracks, gaps, and other unintentional openings in the building envelope. It happens because the pressure across the envelope is not equal—driven by wind, the stack effect, or mechanical system operations—creating an interior-exterior pressure difference. When conditions cause interior pressure to be lower than outside, outside air is drawn in; when interior pressure is higher, air may exfiltrate outward. The key point is that this inward air flow is not by design and is separate from deliberate ventilation. It’s different from ventilation (intentional air exchange for comfort or IAQ), fenestration (the openings themselves), and IAQ (the quality of the indoor air).