In 2018, a University of Hawaii study compared green and white roofs in Honolulu; the green roof was reported to have lower peak temperatures at the surface and nearby air

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.

How the experiment was set up

The researcher built two planter boxes side-by-side on top of an 11-story building in downtown Waikiki, covering a total of 64 square feet, a modest, pilot-scale test plot, not a full building retrofit. She filled them with drought-tolerant succulents called sedums, which are well suited to hot, windy rooftops and need little water. Temperature and humidity readings were collected every 30 minutes for 36 days in the summer of 2018, at three heights: the roof surface, 2 feet above it, and 4 feet above it, to see how far the cooling effect reached.

What the numbers showed

The results were clear. At the surface, the white roof reached a peak of 113°F, while the green roof peaked at 94.5°F, an 18.5°F difference. At 2 feet above the surface, the peak air temperature above the white roof was 107.6°F, compared with 90.9°F above the green roof, a 16.7°F difference. Averaged across the full 24-hour cycle rather than just the peak, the green roof stayed about 2°F cooler than the white roof. At 4 feet above the surface, the gap nearly disappeared, since air at that height mixes more free. However, it's worth noting this is a single 64-square-foot plot, so the finding speaks to the immediate air just above the vegetation rather than any wider street-level cooling. The thesis also found that the green roof took longer to heat up than the white roof, reaching peak temperature one to three hours later, suggesting the soil layer and plants acted as insulation.

Why plants cool things down

Plants are effective at lowering temperatures through a process called evapotranspiration. Through evapotranspiration, plant roots absorb water from the soil and release it as vapor through the leaves. Because turning water into vapor requires energy, heat is drawn away from the surrounding air, much like sweat cools the human body. Green roofs also serve as insulators for buildings and reduce the amount of heat transferred from the roof into the building's interior spaces.

This finding is not unique to Hawaii. Studies carried out using rooftop approaches in the Baltimore-Washington Metropolitan area indicated that as more roofs are transformed to green roofs, surface and air temperatures in urban areas decrease consistently, as was seen in the study titled ‘The effectiveness of cool and green roofs as urban heat island mitigation strategies’ published by Environmental Research Letters. The paper used the WRF model coupled with the Princeton Urban Canopy Model to test the Baltimore-Washington region during the 7–10 June 2008 heat wave, and found the surface urban heat island fell more sharply than the 2 m air-temperature signal as roof coverage increased. The authors report that roughly 30% green-roof coverage, or cool roofs with albedo 0.7, could cut the peak surface UHI by 1°C, while about 90% to 95% coverage was needed to trim the near-surface UHI by 0.5°C. Another study titled ‘Green Roofs in the New York Metropolitan Region’ conducted by Columbia University and NASA found that green roofs could help reduce summer heat in cities and ease pressure on stormwater infrastructure.

What this means for hot, growing cities

Honolulu isn’t alone in experiencing rising temperatures during summer. Around the world, people are looking for cost-efficient and easy solutions for cooling that do not require any major construction. While green roofs cannot be expected to be a magic fix for the issue of urban heat alone, the results from the little experiment on the roof show that they are likely to perform better than those widely used white roofs. In Honolulu, where sunlight is abundant for much of the year, that extra cooling could matter. Dionne's thesis notes that rooftops are an untapped asset in most urban areas and often go unused except for solar panels. Further experiments involving deep soil layers and varied plants can help green roofs play a larger role in how tropical and subtropical cities push back against increasing urban heat, one rooftop at a time.