The effectiveness of white painted roofs against heat in cities has been confirmed.

A technique that has been tested in many cities, repainting roofs and structures white, is often presented as an effective remedy against urban heat waves. Two recent studies confirm the beneficial effect of this solution, which can reduce ambient temperatures by up to 2 degrees Celsius.

However, white paint would be more effective than other alternatives such as solar panels, which are placed on roofs to create shade and on the ground to absorb solar radiation, or vegetation, according to scientists from University College London (UCL).

Their study, published Thursday in the journal Geophysical Research Letters, was conducted using 3D climate models of Greater London using data from the summer of 2018, the hottest summer ever recorded in the British capital.

“If implemented on a large scale across London,” it shows, cooling roofs painted white or made of reflective material (also called “cool roofs”) could “reduce the city’s outside temperature by 1.2 degrees and up to 2 degrees in some places.”

“Cool roofs are the best solution to keep temperatures down on the most extreme summer days. Other methods have additional beneficial effects, but none of them have succeeded in reducing heat in a similar way,” says Dr. Oskar Brus, lead author of the study.

Better than revegetation

Solar panels or greening the streets would only lower the temperature by 0.3 degrees on average.

In the same way, green roofs, in addition to being difficult to implement (the necessary support for the additional weight they represent) and despite their benefits in weather drainage or biodiversity, would only have a “minimal impact” on recovery, for example. the study authors.

Finally, air conditioning, by transferring heat from inside buildings to outside, would warm the city as a whole by 0.15 degrees, but up to +1 degree in central London, contributing to the formation of heat islands.

“By returning heat rather than absorbing it, ‘cool roofs’ have the dual advantage of lowering the temperature not only in the outdoor urban environment, but also inside buildings,” the study asserts.

Redraw everything?

Another study from March, this time conducted in real-world conditions in an area of ​​Singapore that had its roofs, walls and road surfaces repainted white, showed that overall afternoon temperatures could drop by up to 2 degrees, and that thermal comfort for pedestrians improved by up to 1.5 degrees.

This is due to the “albedo effect”, where the lighter the surface, the more light, and therefore heat, it reflects. For example, a white roof made of thermoplastic can reflect 80% of the sun’s rays.

These methods, recommended by the Intergovernmental Panel on Climate Change and already practiced in several Mediterranean countries, such as Greece, are being tested in different cities in countries that were previously temperate, but are increasingly experiencing heat waves.

Could all-white cities be the solution to the heat?

Depends. For those who have chosen to cover buildings, but also floor coverings in a solid color, the results seem mixed.

And so in Los Angeles, which repainted part of its sidewalks white in 2017, passersby complained of feeling hotter — heat only drops at ground level, not at human height — and the dazzling effect of the white.

Leon also ended a two-year street test of the white paint in August, deeming it too expensive and too messy.

On the contrary, when it is used only on the roofs of buildings, the experience is conclusive: in Grenoble, after repainting the Bifork, the assembly hall, in white, the city is enthusiastic about the results, -4 degrees according to an independent study.

In Tremblay-en-France, the Jean-Guimer gymnasium, which will serve as a training centre for the Paris 2024 Olympics, has gained 5 degrees cooler after its roof was cleaned.