Honolulu gets pretty hot, and like most cities, much of that heat comes from rooftops absorbing sunlight all day. In 2018, Elizabeth Dionne, a graduate student pursuing a Master of Geoscience for Professionals at the University of Hawaii at Mānoa, had an idea and decided to put it to the test. She built a small green-roof test plot on the rooftop of the Equus Hotel in Waikiki, next to the building's existing white roof. What followed was a 36-day observation of the temperature difference between the two rooftops. Her thesis findings were striking: at the highest temperature point, the green roof was 18.5°F cooler than the white roof right next to it. At two feet above it, the air was 16.7°F cooler as well.
Why rooftops matter more than we think
Urban areas trap heat. With concrete, asphalt, and dark-colored roofs, the sunlight gets absorbed throughout the day, but it is released slowly at night. This phenomenon is known as an urban heat island, and it makes cities warmer than the surrounding rural areas. Dionne's thesis notes this effect has been associated with increased energy consumption, poor air quality, and adverse effects on people's health. Painting roofs white has been helpful for many urban areas since light colors reflect sunlight rather than absorb it. This small pilot project aimed to test whether a green roof could be more effective than a white roof.
