Why Are the French Painting Their Windows with Chalk This Summer?

As record-breaking heat sweeps across France, shops are running short of an unlikely product: crushed chalk.

Known as Blanc de Meudon, it is normally used for cleaning or paint, but people are now painting windows with chalk to fight punishing indoor temperatures.

The trend has spread quickly on social media, and research suggests there may be genuine science behind this DIY cooling hack.

What Is Blanc de Meudon and Why Is It Selling Out?

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

Demand has soared since the chalk-paint trick spread online, causing stock shortages nationwide as temperatures crossed 40C.

How Does the Cooling Effect of Chalk Work?

The cooling effect comes down to calcium carbonate, chalk's main component. According to Xiangyu Li of Purdue University's Cooling Technologies Research Centre, chalk absorbs very little sunlight and barely any UV or near-infrared light, making it highly reflective, as per the BBC.

Is This Just Another Version of White Paint?

White paint is already known for helping reduce heat gain on walls and roofs, but typical commercial heat-rejecting paints reflect about 80% to 90% of sunlight and usually do not cool below ambient temperature.

Purdue University researchers developed an ultra-white paint that reflects up to 98.1% of sunlight and has been shown in tests to keep surfaces below surrounding air temperature under direct sun.

Could Painting Windows With Chalk Really Lower Indoor Heat?

Painting windows white can significantly reduce indoor temperature, with a stronger effect than painting walls.

Schools and households across France have applied the mixture directly onto glass, creating a milky coating that still lets some light through.

Is This Cheaper Than Air Conditioning?

Chalk and white paint are inexpensive and use energy only during production, unlike air conditioning, which consumes power and releases heat outdoors.

What Is 'Le Cool Roofing' and How Does It Connect to This Trend?

White-painted cool roofs, known in France as "le cool roofing," are gaining attention as a low-tech defence against extreme heat, echoing a long Mediterranean tradition of painting homes white.

Are There Any Downsides to Using Chalk Paint?

Chalk is considered relatively safe, though there may be some respiratory risk from inhaling particles indoors.

One official also cautioned that it is "not a miracle solution" and proper roof insulation remains essential, as per the BBC.

As heatwaves intensify, the appeal of windows with chalk lies in its simplicity. The cooling effect of painting windows may not replace insulation, but it offers an affordable, low-tech buffer against extreme heat.

(With inputs from yMedia)