Why French people are painting chalk on their windows in the heat

A simple, low-tech remedy may help cool homes. Here's the science behind the trend.

As record-breaking heat sweeps over France, some shops are running out of a simple, cheap and unexpected product – crushed chalk.

Known as Blanc de Meudon, or Meudon whiting, it is normally used to make paints or as a cleaning product. But faced with punishing temperatures, there are reports that ingenious people have been using the chalky material as a home remedy against the heat, covering windows in schools and private homes.

Mixed with water, then painted on glass, the result is a milky, whitish coating that lets in some light but reflects the heat.

And a growing body of research suggests that there may be some solid science behind the DIY cooling hack. With heatwaves growing ever more frequent and intense due to rising global temperatures – and posing a particular danger to populations in cities – could a simple lick of white paint help people cope better when it hots up?

White paint – on walls and roofs, usually – is widely known to have a cooling effect. Generally speaking, white surfaces reflect sunlight and heat, while dark surfaces absorb it. This principle can be used to cool buildings and cities.

One study suggests that cool roofs – roofs painted white or with reflective coating – could have cooled London "by about 0.8C (33.4F) on average during a heatwave, preventing the heat-related deaths of an estimated 249 people". (Learn how women in India are using white paint to cool their homes.) 

Paint specially developed to maximise cooling, such as ultra-white paint, has been shown to cool the indoor temperature by several degrees by not only reflecting sunlight but also shedding heat through a process know as "radiative cooling".

One study of ultra-white paint found it could reflect up to 98.1% of sunlight, while a previous formulation reflected 95.5% of sunlight. Reflective paint was found to reduce the temperatures inside by at least 1.7C (3F) compared to the ambient temperature at noon.

Another study showed that combining it with a layer of ultra-black paint underneath could lower daytime temperatures by up to 7.6C (14F).

Chalk has also........

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