Building enclosures should be designed and constructed to be as airtight as possible. This statement confuses many people. They ask: “Shouldn’t buildings breathe?” “Don’t tight buildings make people sick?” “If you tighten up a building enclosure, won’t you trap the interior moisture and keep it in?” These questions show a lack of understanding about water vapor diffusion through building materials and its control; the buffering of moisture by building materials, and air leakage and its role in transporting moisture through the cavities of the building enclosure; and the control of air leakage using air barrier systems.