Making roofs white or reflective is the best way to keep a city cool

Painting rooftops white or covering them with a reflective coating is the best way to reduce air temperature in a city like London during a heatwave. These “cool roofs” should perform better than solar panels, green roofs or adding more trees at ground level.

Oscar Brousse at University College London and his colleagues ran climate simulations to see how London’s temperature during the two hottest days of summer in 2018 would have changed if the city had made widespread use of cooling measures, from cool roofs to air conditioning, along with solar panels, which have a cooling effect. Temperatures that summer peaked at 35.6°C (96.1°F).

Cool roofs outperformed all other interventions, the team found, lowering average outdoor temperatures in the city over the two-day period by 1.2°C, and by as much as 2°C in certain locations. By comparison, additional tree cover only curbed air temperatures by about 0.3°C, while solar panels lowered temperatures by 0.5°C.

The study also found that widespread use of air conditioning might keep internal temperatures cool, but would boost outdoor air temperatures by up to 1°C in parts of central London. “For London, what worked the most for reducing outdoor temperatures at a pedestrian level was the cool roofs,” says Brousse.

Although the study uses only two days of data for the simulations, Brousse says it is broadly in line with similar published research.

Installing reflective coatings, or lightening roof surfaces across the city, would be a relatively easy and low-cost climate adaptation, says Brousse. “I think, actually, this is probably the most easily deployed intervention of all,” he says. “I see barely any reason not to start doing it widely.”

There are reasons beyond cooling to deploy other technologies too, he says. Trees and green roofs can boost biodiversity and resident well-being, for example, while solar panels provide clean power.

A 2023 report by the Greater London Authority suggested that cool roofs may become an increasing policy focus for city officials as summer temperatures rise under climate change. “With temperatures in London projected to increase, and with more occurrences of heatwaves, reflective roofs are likely to be a key component of climate adaptation strategy,” the report says.