As another grueling summer heat wave hits France, many residents are turning to a unique, low-cost trick to make their homes cooler — coating their windows with Blanc de Meudon. The chalk-and-water coating offers a cheaper way to block heat than buying an air conditioner and covering the cost of running it.
What's happening?
Demand has risen in France for Blanc de Meudon, a powdered chalk traditionally used in cleaning and paint preparation, according to The Economic Times.
When mixed with water and brushed onto glass, it leaves a pale layer that reflects sunlight outward instead of letting rooms absorb that heat.
Popularity is exploding for this hack — some reports said retailers were even running out of the product.
Xiangyu Li, who researches cooling technologies at Purdue University, told the BBC: "Chalk is mostly calcium carbonate. It absorbs very little sunlight, including the visible range, making it white. It also absorbs very little ultraviolet and near-infrared light, making it highly effective at reflecting sunlight."
Calcium carbonate is also being examined for more sophisticated cooling materials. Jiashuo Wang, a researcher at the University of New South Wales, said these particles are commonly used in advanced radiative cooling coatings because they reflect so effectively.
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Why does it matter?
Price is a major part of the appeal.
Since the coating keeps working without electricity once it is applied, households may be able to use air conditioning less and not rack up their power bill as much. For homes without AC, it could also provide some relief at a much lower cost than buying a new system.
As extreme heat becomes more common, staying cool is getting pricier for many households. Air conditioners can cool spaces quickly, but they also draw large amounts of electricity and release waste heat outside, which can intensify urban heat in densely built areas.
Experts say chalked windows are not a substitute for stronger insulation, reflective roofs, or better building design. Even so, Li noted that windows are a key point of focus because they account for a meaningful share of solar heat entering a home.
What's being done?
Researchers and cities are trying out other versions of this same reflective strategy.
Reflective white roofing, often referred to as cool roofs, is drawing interest because it can reduce heat across whole neighborhoods rather than only inside individual buildings.
One study found that widespread use of cool roofs in London could have lowered average temperatures during a heat wave by about 1.4 degrees Fahrenheit (0.8 degrees Celsius) and prevented roughly 249 heat-related deaths. The Economic Times also highlighted Purdue research on an ultra-white paint that can reflect as much as 98.1% of sunlight, far more than standard white paint.
Scientists are also exploring coatings for fabrics and less conventional materials for windows. In the United Kingdom, researchers found that applying a thin yogurt layer to windows reduced indoor temperatures by an average of 1.1 degrees Fahrenheit (0.6 degrees Celsius), with drops reaching 6.3 degrees Fahrenheit (3.5 degrees Celsius) in hot, sunny conditions.
Other lower-cost measures — including reflective window coverings, shading, sealing leaks, and cool-roof products where available — can also help keep indoor spaces cooler and may lower energy expenses.
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