Infiltration is the unintentional or accidental introduction of outside air into a building, typically through cracks in the building envelope and through use of doors for passage. Infiltration is sometimes called
air leakage. The leakage of room air out of a building, intentionally or not, is called
exfiltration. Infiltration is caused by
wind, negative pressurization of the building, and by
air buoyancy forces known commonly as the
stack effect.