London study finds cool roofs beat greenery to help manage urban heat

A new study by University College London (UCL) researchers suggests painting roofs white or covering them with reflective coatings could be the most effective way to cool cities like London during scorching summer days.

The research, published in Geophysical Research Letters , compared the effectiveness of various passive and active urban heat management systems. This included “cool roofs,” rooftop solar panels, green roofs, increased street-level vegetation, and air conditioning. The team used a detailed computer model to simulate these methods during the two hottest days of London’s record-breaking summer in 2018.

The study found that widespread adoption of cool roofs could reduce outdoor temperatures across the city by an average of 1.2 degrees Celsius (2.16 degrees Fahrenheit), with some areas experiencing up to a 2-degree Celsius (3.6-degree Fahrenheit) drop.

“We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures, and found that cool roofs were the best way to keep temperatures down during extremely hot summer days,” Dr. Oscar Brousse, of UCL’s Bartlett School of Environment, Energy & Resources and lead author of the study, said in a statement .

While other methods offered environmental benefits, their cooling effects were less noticeable. Extensive tree cover and solar panels, for example, only provided an average city-wide cooling effect of about 0.3 degrees Celsius (0.54 degrees Fahrenheit). Green roofs, lauded for their water management and wildlife habitat benefits, showed a negligible net cooling effect overall.

The study also highlighted the potential drawbacks of air conditioning. While it cools buildings, it expels heat outdoors, further warming the urban environment. The researchers found that extensive air conditioning use could raise outdoor temperatures by an average of 0.15 degrees Celsius (0.27 degrees Fahrenheit) across London, with a more significant increase of 1 degree Celsius (1.8 degrees Fahrenheit) in dense central areas.

However, the study suggests this increased air conditioning demand could be entirely powered by rooftop solar panels if installed on a similar scale.

A sustainable approach must be holistic

The research team emphasizes the importance of considering the full impact of each method. Cool roofs, by reflecting sunlight, offer the dual benefit of cooling both the urban environment and building interiors.

While green roofs didn’t offer a significant overall cooling effect, the study did find variations throughout the day. During peak heat hours, widespread green roofs could lower temperatures by 0.5 degrees Celsius (0.9 degrees Fahrenheit). However, this benefit was offset at night, as the roofs retained heat and released it after sunset, raising nighttime temperatures by a similar amount.

The study suggests that converting urban green spaces from grass to deciduous trees might provide some nighttime cooling but could have mixed effects during the day. Additionally, increased tree cover could raise air humidity, potentially impacting resident comfort.

This research offers valuable insights for city planners and designers grappling with rising urban temperatures. By comprehensively comparing various cooling strategies, the study highlights the potential of cool roofs as a powerful tool for mitigating urban heat and improving resident comfort during extreme heat events.