Infiltration/exfiltration is the uncontrolled inward/outward leakage of outdoor/indoor air through cracks, interstices and other unintentional openings of a building, caused by the pressure effects of the wind and/or the stack effect [1].

[1] M. Limb, “Technical note AIVC 36- Air Infiltration and Ventilation Glossary,” International Energy Agency energy conservation in buildings and community systems programme, 1992.