Energy-efficient buildings are not living up to their promise. The problem is serious, yet somewhat puzzling. Efficient building technologies are becoming more and more available; government regulations and company policies are supporting a drive for energy efficiency more than ever and building energy performance targets remain low. Yet when high-performance buildings are delivered they are not matching expectationsThe reasons for the performance deficit can be attributed to multiple issues. Building system and envelope modeling may be inadequate, possibly hampered by incomplete application of the tools—probably because they are not calibrated to actual operating conditions. Contractors may understand each building system