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Study: Painted Roofs Cool More than Green Roofs


New research from the University College London has reportedly found that painting roofs white or using a reflective coating is more effective at cooling cities than using “green roofs,” street-level vegetation or solar panels.

Additionally, the researchers found that extensive use of air conditioning warmed the outside environment by as much as 1 degree Celsius (1.8 degrees Fahrenheit) in London’s city center.

About the Research

According to the university, as the climate changes, residents within cities are particularly vulnerable to warming temperatures due to the Urban Heat Island effect. To adapt to or mitigate these temperatures, city planners and designers have looked at passive cooling methods like cool roofs, green roofs and expanded urban vegetation, as well as active methods for building interiors like air conditioning.

The study offered additional insights into urban heat management by comprehensively comparing several common passive and active heat mitigation actions.

To offer additional insights into urban heat management, the research used a three-dimensional urban climate model of Greater London to test the thermal effects of different passive and active management systems.

This included painted “cool roofs,” rooftop solar panels, green roofs, ground level tree vegetation and air conditioning during the two hottest days of the summer of 2018, the warmest on record.

To gauge the potential full effect of each method, the team reportedly modeled each one as though they had been as widely adopted as theoretically feasible across housing, commercial and industrial buildings throughout Greater London.

“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,” said lead author Dr. Oscar Brousse from the UCL Bartlett School Environment, Energy and Resources.

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“Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”

Researchers reportedly found that, if widely adopted throughout London, cool roofs could reduce outdoor temperatures across the city on average about 1.2 C and up to 2 C in some locations. By reflecting rather than absorbing heat, cool roofs could also decrease the temperatures inside of buildings as well.  

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However, other systems like street-level vegetation or solar panels only provide a smaller net cooling effect of about 0.3 C despite other environmental benefits. The study notes that green roofs’ net cooling effect was found to be “negligible” on average.

Despite this, the research team reportedly found that their effect on temperature varied “significantly” throughout the day. During the warmest times of day, the wide adoption of green roofs could lower urban temperatures by an average of 0.5 C; this would be offset overnight as the thermal mass from the roofs would retain daytime heat, releasing when the sun was down and increasing night-time temperatures by about the same amount.

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Air conditioning would also reportedly warm the outdoor urban environment by about 0.15 C for the city overall, but by as much as 1 C in central London. The researchers add that the increase in the distribution of air conditioning units in their model could be entirely powered by photovoltaic solar panels if they were similarly installed to their fullest extent.

According to the study, converting city greenspaces from grass to deciduous tree cover would cool temperatures overnight but at best would have mixed net effects during the day. Additionally, it would likely increase the amount of water vapor in the air and increase the air humidity, affecting residents’ thermal comfort.

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The research was supported by Wellcome and NERC. The results were recently published in the journal Geophysical Research Letters.


Tagged categories: Colleges and Universities; Cool Coatings; Cool roof coatings; Green roofs; Reflective coatings; Reflective roof coatings; Research and development; Roof coatings; Roofs; Temperature


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