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CONTROLLING MOISTURE in Mixed Climates Details for keeping rain and water vapor out of wall cavities Controlling moisture problems is difficult in climates where seasonal changes necessitate both heating and cooling of interior spaces. In addition to common concerns about rain penetration through exterior cladding, builders in a mixed climate must take special precautions to prevent moisture from becoming trapped in wall and roof cavities — a problem that is complicated by high humidity levels both inside and outside the structure. Infiltration & Exfiltration During heating periods, when interior air pressure is high, humid air inside the building is driven outward, either by air leaks into wall and roof cavities (exfiltration) or by diffusion (passage of water