A BUILDING IS A RATHER COMPLEX AND LONG-lived system. Its shell, building enclosure (BE), “breathes,” ages, expands, contracts, and is exposed to an ever-changing set of conditions induced by nature and man. Sometimes the people change the fabric from the original design; buildings are reinsulated, renovated, and rehabilitated. The designer must consider all aspects of the building's life during the design and yet keep the building affordable and competitive. The design of the building enclosure aimed in addressing the set of interacting conditions in heat, air, moisture flows, and their effects on construction, is called “environmental control.”