France adopts chalk‑coated windows to cool homes during historic heatwave

Amid an unprecedented heatwave across France, residents have turned to a low‑cost DIY method: mixing Blanc de Meudon chalk powder with water and painting the mixture on windows. The white, milky coating reflects a large portion of solar radiation, reducing indoor temperatures. Scientific explanations cite calcium carbonate’s ability to reflect ultraviolet and infrared light, and studies show white surfaces can lower building temperatures by at least 1.7 °C, with some formulations achieving drops of up to 7.6 °C. Research on similar reflective roofing in London suggests that widespread adoption could prevent hundreds of heat‑related deaths.

The practice spread rapidly through social media, prompting surges in chalk demand that left shelves empty in many regions. Schools and households alike have applied the solution, viewing it as an energy‑light alternative to air‑conditioning, though experts note it is not a substitute for proper insulation. The trend highlights how simple, locally available materials are being used to mitigate extreme heat as climate‑driven temperatures rise.