This paper discusses prevention of interstitial condensation in masonry walls due to infiltration of warm moist air into air-conditioned buildings in summer and exfiltration of warm moist air in winter. The following aspects of the problem are discussed: air pressure differentials due to wind, stack effect, and mechanical ventilation; air leakage, modes, and test methods; climate effects; air barriers, insulation materials, and their placement in cavity walls, veneer walls, and single wythe walls; methods for sealing cracks and joints; and building code requirements. Thirty-one references are cited.