Air leakage is sometimes called infiltration, which 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. In the summer, infiltration can bring humid outdoor
air into the building. Whenever there is infiltration, there is corresponding exfiltration
elsewhere in the building. In the winter, this can result in warm, moist indoor air moving into
cold envelope cavities. In either case, condensation can occur in the structure, resulting in
mold or rot. Infiltration is caused by wind, stack effect, and mechanical equipment in the
building. Wind creates a positive pressure on the windward face and negative pressure on the non-windward (leeward) facing walls, which pulls the air out of the building. Wind causes infiltration on one side of a building and exfiltration on the other. Wind effects can vary by surrounding terrain, shrubs, and trees.
