White roofs outperform green roofs and solar panels in urban cooling

New research from UCL reveals that painting roofs white or covering them with reflective coatings is the most effective method of cooling cities like London, beating green roofs, street-level vegetation and solar panels

According to research by UCL scientists, painting roofs white or covering them with reflective coatings is more effective in cooling cities like London than green roofs, street-level vegetation, or solar panels. Additionally, extensive use of air conditioning can raise the external temperature by up to 1.8°F (1°C) in the densely populated center of London, the researchers discovered.

The study, published in Geophysical Research Letters, utilized a 3D urban climate model of Greater London to test the thermal effects of various urban heat management systems, both passive and active. These included “cool roofs” (painted white), rooftop solar panels, green roofs, street-level vegetation, and air conditioning during the two hottest days of the summer of 2018, the hottest on record.

White roofs: results and environmental benefits

The study revealed that if widely adopted across London, white roofs could reduce external temperatures by an average of 2.2°F (1.2°C), and up to 3.6°F (2°C) in some areas. Other systems, such as extensive street-level vegetation or solar panels, offered a smaller net cooling effect, around 0.5°F (0.3°C) on average, although they provide other environmental benefits. Similarly, while green roofs offer advantages like water drainage and wildlife habitat, their net cooling effect on the city was found to be negligible on average.

Previous studies have shown that white roofs are three times more efficient than green roofs in keeping buildings cool, reflecting sunlight and significantly reducing both internal and external temperatures.

A noteworthy example is an initiative in Bermuda, where a company has introduced white solar panels integrated with white roofs. This solution not only meets aesthetic needs but also improves the energy efficiency of the panels by reflecting sunlight, enabling better performance in high-heat conditions. This innovation could extend to other regions, further promoting the adoption of integrated urban heat management solutions.

Air conditioning, which transfers heat from inside buildings to the outside, could raise the urban environment temperature by about 0.3°F (0.15°C) overall, but up to 1.8°F (1°C) in central London. The researchers also found that increasing the distribution of air conditioning units in their model could be entirely powered by photovoltaic solar panels if installed to their maximum extent.

To evaluate the full potential effect of each method, the team modeled each as if they were adopted as widely as possible in residential, commercial, and industrial buildings throughout Greater London. The lead author of the study, Dr. Oscar Brousse (UCL Bartlett School Environment, Energy & Resources), stated:

“We comprehensively tested various methods that cities like London could use to adapt and mitigate rising temperatures, and we found that white roofs were the best way to keep temperatures low during extremely hot summer days. Other methods had various important side benefits, but none could reduce urban heat to the same extent.”

Climate change impacts and possible solutions

With climate change underway, residents in cities are particularly vulnerable to rising temperatures. Cities tend to trap heat, inducing the Urban Heat Island effect, which can lead to increased discomfort and mortality during heatwaves. Finding ways to adapt to or mitigate these rising temperatures has become a priority for urban planners and designers, who are exploring passive cooling methods like white roofs, green roofs, and expanding urban vegetation, as well as active methods for building interiors such as air conditioning.

The study provided further insights into urban heat management by comprehensively comparing various common heat mitigation actions, both passive and active. By reflecting rather than absorbing heat, white roofs offer the dual benefit of cooling not only the external urban environment but also the interiors of buildings.

Source: UCL