As climate change brings more extreme heat, keeping homes cool is becoming increasingly important—especially for vulnerable communities. While we know that air conditioning (AC) helps protect people during heatwaves, there hasn’t been a good way to measure exactly how much it cools indoor spaces on a large scale.

To solve this, we used simulations of 11,700 typical homes across Massachusetts—representing about 3 million actual buildings. By combining detailed building models with weather data, we estimated how hot it gets inside homes with and without AC or energy-saving upgrades like insulation.

We found that AC lowers indoor temperatures by about 6.5°C (12°F) on average, while weatherization alone made only a small difference (around 0.1°C (0.2°F)). Some neighborhoods were much hotter than others, but AC helped narrow those differences when compared to those without AC.

Our approach offers a new way to estimate indoor heat across entire states. It also shows that expanding access to AC and improving home insulation could protect people from dangerous heat more effectively.