Repainting rooftops or entire buildings in white is often presented as an effective remedy against urban heatwaves. Two recent studies confirm the beneficial effect of this solution, which could reduce the surrounding temperature by up to 2 degrees. The first study, conducted by scientists from University College London, was based on data from the summer of 2018, the hottest ever recorded in the British capital.
It shows that « if implemented on a large scale throughout London », white or reflective material cool roofs can « reduce the outdoor temperature in the city on average by 1.2 degrees and up to two degrees in some areas. » « By reflecting heat rather than absorbing it, cool roofs have the double advantage of reducing the temperature not only in the external urban environment but also inside buildings, » the study points out.
Solar panels, which create shade on rooftops and absorb solar radiation on the ground, or greening streets, despite their benefits for weather drainage or biodiversity, would only reduce the thermometer by an average of 0.3 degrees, according to 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.
Successful test in Grenoble but not in Lyon
Another study from March, this time carried out in real conditions in a district of Singapore where rooftops, walls, and road surfaces were painted white, showed that the overall temperature could be reduced by up to 2 degrees in the afternoon, and that the thermal comfort experienced by passersby was improved by up to 1.5 degrees. This is due to the « albedo effect », whereby the lighter a surface, the more it reflects light, and therefore heat. A white thermoplastic roof, for example, can reflect 80% of the sun’s rays.
These methods, recommended by the IPCC and already practiced in several Mediterranean countries like Greece, are being experimented in different cities in formerly temperate countries but now facing increasingly regular heatwaves. Would entirely white cities then be the solution to heat? The assessment is mixed. For example, in Los Angeles, which had repainted a part of its sidewalks white in 2017, pedestrians complained of a greater sense of stuffiness – the heat being attenuated only at ground level and not at head height – and the dazzling effect of white. Lyon also ended a two-year test of white surfacing in the street in August, judging it too expensive and too dirty.
Conversely, when used only on building rooftops, the experience is successful. In Grenoble, after repainting the Bifurk, an associative hall, in white, the city is enthusiastic about the results, -4 degrees according to an independent study. In Tremblay-en-France (93), the Jean-Guimier gymnasium, which will serve as a training center for the Paris 2024 Olympics, gained 5 degrees of cooling after making its roof immaculate.
