TRUE OR FAKE. Heat wave: can the temperature of a roof drop by 30 degrees with white paint, as Eric Piolle claims?

What if all it took was a coat of white paint to stop the heat wave? While the Mediterranean rim is experiencing temperature records in July, the cities of southern Europe, affected by heat island phenomena, have turned into a furnace. In Grenoble, where the thermometer more and more often exceeds 40°C, the roof of the Bifurk, ​​a former industrial building converted into an associative nursery, was painted white in order to fight against this phenomenon.

A successful operation, according to the mayor of Grenoble Eric Piolle. Bifurk roof temperature dropped 30°C with white paint”, he rejoiced on Twitter on July 21. A study is in progress on other municipal buildings” and streets have also been renovated on the same principle using “light materials”specifies the city councilor in another message.

Does the idea actually make it possible to fight against the heat wave? Franceinfo asked the question to experts.

25 to 30°C difference at summer peaks

In Greece, Italy or Morocco, white walls and roofs are part of the traditional architecture of cities. Empirically, the inhabitants of these very sunny countries understood that painting their buildings in white allowed to protect themselves from the heat, advances Tangui Le Dantec, architect and teacher in environmental engineering. “When light energy, which comes from the sun, hits a surface, it is either reflected or converted into heat. thermodynamics”develops this expert.

Therefore, the more a surface reflects light, the less heat it stores, and vice versa. This phenomenon is called the albedo effect. According to this principle, a mirror reflects 100% of the light, while a perfect black body would totally absorb it. White paint reflects about 80% of the light – it is for this reason that our eye perceives it as white. But is this enough to allow a drop in temperature as significant as that advanced by Eric Piolle?

To find out for sure, the town hall of Grenoble commissioned the Canopée design office, which specializes in the environmental quality of buildings, to assess the performance of the white paint applied to the roof of the Bifurk. Its report, which BlazeTrends was able to consult, concludes that the device is effective and considers that “the process could therefore be extended to other buildings in the agglomeration”. However, the figure of 30°C put forward by Eric Piolle should be put into perspective.

In its file, the firm compares the temperatures recorded before and after the application of the white paint. In 2020, before the roll stroke, “the roof temperature reached almost 70°C”. In 2021, once the roof has been repainted in white, “the roof surface temperature has never reached 50°C”with peaks around 45°C, which makes it possible to conclude that “average difference of about 25°C to 30°C” at the height of summer. But when you consider the average, however, the measurements are a bit less impressive. The temperature measured at the surface of the roof averaged “32.1°C” in 2020, against “22.7°C”in 2021, a difference of 9.4°C.

Moreover, these measurements were not carried out in the same year. The weather was therefore not exactly the same in the summer of 2020 and the summer of 2021. According to the Infoclimat website, the summer of 2021 was a little less hot than the summer of 2020. In 2020the average temperature was 24.5°C in July and 24°C in August. While in 2021, it was a little cooler with an average of 22.5°C in July and 21.8°C in August. On the maximum side, the thermometer recorded 40.6°C in the summer of 2020, while in 2021, the mercury never exceeded 36°C, according to readings from the Saint-Martin-d’weather station. Hères (Isère). Part of the drop in temperature observed on the roof is therefore attributable to the weather, according to the report.

A cooling effect in and around the building

The statements are formal, the surface of the roof is less hot with the white paint. But what about the air inside the building? Still according to the Canopée report, “the average indoor temperature has decreased by 4.3°C” between summer 2020 and 2021. However, “this reduction results both from the treatment of the roof and from the climatic conditions”, recall the authors of the report. To correct this bias, the research department compared the difference observed between outdoor air and indoor air for the year 2020 and 2021. The report thus concludes with a reduction in the average difference between indoor temperature and the outside temperature “by 1.1°C” From one year to another.

Concretely, in 2020, it was on average 4.4°C warmer in the Bifurk, ​​compared to the air temperature outside the building. While in 2021, it was only 3.3°C warmer inside the associative nursery, compared to the outside air. The white roof therefore limits the heating of the air during hot weather. However, “this process does not prevent a building like the Bifurk from overheating in the medium term during the summer period”, notes the report, since the air stays warmer inside than outside.

The difference may therefore seem marginal, but a difference of one degree can have a significant impact during a heat wave. This is especially true at night, when every degree counts to prevent heat-related deaths. Indeed, the white roof is not only beneficial for the air inside the building. According to Tangui Le Dantec, going from a black asphalt roof to a white roof, “allows to lower the local heat island, with a visible effect at night when the temperature drops by several degrees”. Indeed, the black asphalt, which absorbs a lot of heat, also stores it over time. And at night, this heat continues to diffuse.

A solution not suitable for pavements

So should we start painting everything white to fight against the effects of climate change? In Los Angeles (United States), the municipality has made this bet. Since 2017, the city has been experimenting with “fresh pavements”, i.e. kilometers of roads repainted in white. The effect on islets of heat is, it seems, visible from space. “We saw a difference of 10 to 12 degrees Fahrenheit (5.5 to 6.6°C) between the areas we painted and the areas we didn’t paint at street level”, explain on the NASA website the american physicist Glynn Hulley.

However, if this technique helps combat heat islands, the white streets of Los Angeles can turn into an oven for pedestrians, reports a study conducted by two American researchers. “The light energy that hits a white surface is not dissipated, but reflected. So even if the ground is less hot, when you walk on the pavement, you bake, because it is the human body that absorbs the energy solar”explains Tangui Le Dantec.

He recommends reserving white paint for high roofs, out of sight of pedestrians and other dwellings. “Watch out for the dazzling effect!”abounds Claire Doussard, doctor in planning and specialist in sustainable urban planning issues. “In the public space, white is not a solution. It is better to favor stabilized sands”, she explains. Advice that the city of Grenoble is already applying, especially in the Admirals district. “We put beige aggregates, light gray, but never white on the ground”assures Margot Belair, assistant to Eric Piolle at the town hall of Grenoble, in charge of town planning. “It’s prettier than black and heat builds up less, but it’s more expensive. It’s a political choice.”

Paint the roofs white “is not a miracle solution. It must be part of a panel of solutions”, adds the assistant. Because if the stroke of paint proves effective, it cannot alone counter a heat island. According to specialists, several complementary options appear. Among them: the revegetation of space by increasing the surface of the canopy to obtain shade, the permeabilization of soils by favoring earth over bitumen, or even a reflection on the ventilation of cities, avoiding obstructing the air circulation in the urban areas concerned.

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