Evaluation of Buildings The most common and costly problems associated with the performance of a building envelope are related primarily with excessive air leakage and water intrusion. In particular, windows, doors and curtain walls, as well as their interface with the adjacent wall construction, are determining elements in the performance of the building envelope. Improved standards and design principles have contributed in significantly improving the performance of window; doors and curtain wall systems, whether it is with respect to resistance to water penetration, air leakage resistance, wind load resistance or condensation resistance. The reality, however, is that many buildings of recent construction are still experiencing problems with the in-service performance of installed wind