How ordinary chalk can help reduce heat in your home

Screenshot from video: lhommedemenage / TikTok

While many countries try to escape the heat with air conditioners, the French have remembered an old remedy — Blanc de Meudon — natural chalk ground into a powder. This material is usually used as a pigment for paints or a cleaning agent. Another traditional application is coating glass storefronts during renovation to make them opaque. Gardeners also apply it to greenhouse glass to protect plants from overly bright sun.

But during extreme heat, Blanc de Meudon unexpectedly became popular as a simple way to protect homes from the sun. According to French media, demand for it caused a shortage of the product across the country when the thermometer crossed the +40°C mark.

The recipe is simple: chalk is mixed with water and the resulting mixture is applied to window panes. After drying, a white matte coating forms. It allows enough soft daylight into the room so as not to live in complete darkness, but at the same time creates an effective barrier against infrared radiation, which is the main source of heating.

Scientific studies indicate that this simple method indeed has a scientific basis. White surfaces reflect sunlight much better than dark ones, which absorb heat. According to research, even ordinary white paint can significantly reduce the heating of walls and roofs, and the temperature on their inner side can be at least 1.7°C lower. This is why in many southern European countries, houses are traditionally painted white.

Even better results are shown by modern "ultra-white" paints, specifically designed for cooling. They not only reflect sunlight but also more effectively radiate accumulated heat through a process known as radiative cooling. According to one study, such paint can reflect up to 98.1% of solar radiation.

In the case of chalk, calcium carbonate — its main component — plays an important role. It effectively reflects ultraviolet and infrared radiation, which brings a significant portion of the heat. This is why calcium carbonate particles are used in the development of new cooling coatings.

Unlike air conditioning, this method does not require electricity.

However, one should not expect miracles from this method. Chalk applied to glass only reduces the amount of solar heat entering the room through the glass. The same principle — reflecting rather than absorbing sun rays — is used in larger urban solutions. For example, painting roofs white is gaining increasing attention. In France, this approach is known as le cool roofing.

According to studies, widespread use of such roofs in large cities can significantly reduce air temperature during heatwaves. Research shows: if white roofs were widely used in London, the average temperature during heatwaves could be approximately 0.8°C lower, and the number of heat-related deaths significantly fewer. The principle itself is not new: in Mediterranean countries, white houses have been helping people better endure summer heat for centuries.

For those who were not lucky enough to find chalk, British scientists offer a more unusual method. Researchers found that after drying, a thin layer of ordinary yogurt on glass also helps reflect solar radiation. According to experiment results, in hot sunny weather, this allowed reducing the room temperature by an average of 3.5°C.

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