Overheating inside buildings can reach levels associated with health impacts during heat wave episodes, and is more likely in older buildings despite trade-offs made to achieve energy efficiency in newer buildings. Researchers at the University of Arizona simulated power-failure conditions in residential buildings in Houston and Phoenix, based on models from the Energy Efficiency & Renewable Energy Office of U.S. DOE. Energy efficiency improvements (such as reducing air infiltration) improved the ability of the building to maintain comfortable conditions indoors, while in the older simulated buildings, discomfort reaches a critical level in about 6 hours. Strategies vary in effectiveness in different climates (such that reducing infiltration is more effective in Phoenix than in Houston) highlighting the need for local, rather than nationwide guidelines for improving passive building survivability.