Why the French are painting their windows with chalk to beat the heat ^⁠17%

6/26/2026, 10:00:00 AM

Topics: Earth, Extreme Weather, Heatwaves, Climate Solutions, Climate, Environment, Features

Keywords: Chalk, Heatwave, Cooling, France, Diy, Radiative Cooling, Calcium Carbonate, Cool Roofs

BS Summary: This article contains 30 faulty reasoning types, including Unattributed Quote, Availability Heuristic, and Anecdotal, with Optimism Bias as the most egregious example at 15.1% saturation with 142 hits. Analysis detected 1,731 faulty-reasoning hits from 938 analyzed words, generating a BS Score of 32.4% and a BS Rank of ⁠17% (18,295 of 21,887 articles). This article is better (less manipulative) than 83.60% of the article peer group.

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938 words

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?

Radiative cooling

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.

Commercially available white paint on a surface can reduce temperatures on the other side by at least 1.7C (3F) compared to the ambient temperature at noon.

Paints specially developed to maximise cooling, such as ultra-white paint, have been shown to reduce 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.

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

One of the reasons why chalk might be effective, however, lies in the properties of its main component, calcium carbonate, which is not only highly reflective but also resistant to solar radiation.

This property has led some researchers to use nanoparticles of calcium carbonate in new kinds of "super cool" paint.

"These kinds of particles are widely used in radiative cooling paint, also in our super cool paint," says Jiashuo Wang, a student at the University of New South Wales in Sydney, Australia, who is part of a team working on cooling paint.

Chalk has also been used as a coating for fabric that keeps the wearer cool.

Particles of calcium carbonate – the main ingredient in chalk – are good at reflecting ultraviolet and near-infrared light (the portion of sunlight that transmits heat).

In addition, chalk is considered to be relatively benign in terms of its health and environmental impact – though there may be some risks to respiratory health from indoor chalk use, and inhaling particles.

White windows and 'le cool roofing'

According to French media reports, demand for Blanc de Meudon is leading to stock shortages around the country as people struggle against temperatures of more than 40C (104F).

Blanc de Meudon is traditionally used to whiten shop windows during renovations or by gardeners in their greenhouses.

But after the chalk-paint trick circulated on social media, demand for the product soared, French newspapers report.

"We'd known about the idea for a while, we talked about it during the last heatwave but forgot to buy any," Ouest France quotes a shopper called Philippe.

"Now it's too late!

It's sold out everywhere!"

Some French schools have also used the chalk paint on their windows, though an official warned that it's "not a miracle solution" and that properly insulated roofs are needed instead.

People have also whitened the windows of their apartments.

The chalk, as well as white paint more generally, is cheap.

And unlike air conditioning, which worsens the overall heat effect and emissions problem by consuming energy and releasing heat outside, paint only uses energy when it's produced.

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Why the French are painting their windows with chalk to beat the heat ^⁠17%

Speakers

Jiashuo Wang

Analysis

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Why the French are painting their windows with chalk to beat the heat ⁠17%

Speakers

Jiashuo Wang

Analysis

Biases

Fallacies

Manipulations

Why the French are painting their windows with chalk to beat the heat ^⁠17%