Center for the Built Environment

Extreme heat causes more weather-related deaths in the United States than any other natural hazard, and these events are projected to increase in frequency, intensity, and duration. As a result, it is critical to ensure safe thermal conditions in homes while minimizing excessive cooling energy use. In California, where the median age of homes is 45 years and nearly 40% lack mechanical cooling, this deficiency undermines one of the most essential goals of housing: to shelter people from outdoor weather of a warming planet. We aim to quantify the overheating risk in the housing sector to support the development of public policies related to maximum safe indoor thermal limits and building energy use. Using the ResStock modeling framework, we created over 52,000 building models to represent California's residential housing stock and assessed overheating risks by simulating indoor temperatures and analyzing the energy impacts of adding cooling systems.Our findings reveal significant regional disparities. Southern counties face the highest overheating risk, while coastal areas are less vulnerable due to oceanic temperatures. Inland counties, such as the Sierra Nevada region, are also less affected due to higher altitudes. Approximately 1.6 million homes will require retrofitting with cooling systems, which could increase peak electricity demand by 2%. Our sensitivity analysis showed that increasing the threshold temperature reduces the number of homes subject to overheating risk. These results significantly impact the electricity grid, emphasizing the need to consider passive cooling options like shading and cool roofs or energy-efficient cooling options like fans and evaporative coolers.