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The approach involves covering the exterior of sun-exposed windows with a chalk-and-water coating so some sunlight is reflected away.
Photo Credit: iStock
As another round of extreme heat drives Americans to rely more heavily on air conditioning, a simple cooling trick from France could help keep energy bills in check.
The approach involves covering the exterior of sun-exposed windows with a chalk-and-water coating so some sunlight is reflected away before it can warm the rooms behind the glass.
During hot weather in France, residents have reportedly been seeking out Meudon whiting — a white powder made mostly of calcium carbonate — as a way to help cool their homes. Some reports say it can make a difference of up to 12 degrees Fahrenheit. 
Tom’s Guide highlighted the method after reporting from the BBC.
According to Tom’s Guide, the standard mixture is one part powder to two parts water. The mixture is brushed onto the outside of windows. Once it dries, it leaves a light-colored coating intended to reflect some of the sun’s rays and slow heat buildup indoors.
Meudon whiting — also known as Spain whiting, Champagne whiting, Troyes whiting, and white polishing stone, according to Tom’s Guide — has long been used for polishing silverware and as a component in paints and putty. 
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It has also been applied to greenhouses to reduce glare and heat. As Sortir à Paris noted, per Tom’s Guide, “In favorable conditions, drops of 3 to 7°C [5-12°F] could be expected.”
“It does actually seem to work,” the BBC reported.
Reducing solar heat gain could help limit how often air conditioning needs to run, make fans more effective, and allow residents to set the thermostat slightly higher without sacrificing too much comfort.
Cooling costs can climb quickly during prolonged periods of extreme heat. If Meudon whiting lowers indoor temperatures by even a few degrees, that difference could still add up to savings over several days or weeks of heavy AC use.
For some households, it may be a simpler option than replacing windows, installing exterior shades, or paying for other major home upgrades. Meudon whiting is also biodegradable, according to Tom’s Guide.
There are tradeoffs, however. The coating gives windows an opaque, whitewashed appearance that can block views and reduce natural light.
Anything that helps an air conditioner run less often can reduce electricity costs, even if the exact savings depend on the home, climate, and window exposure.
Other low-cost strategies include keeping blinds closed during peak sun hours, using fans strategically, and opening windows when outdoor temperatures are cooler.
If you do use Meudon whiting, testing the mixture first in one especially sunny room may be a practical way to see whether the temperature drop feels worthwhile.
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© 2025 THE COOL DOWN COMPANY. All Rights Reserved. Do not sell or share my personal information. Reach us at hello@thecooldown.com.

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