Many French households are using Blanc de Meudon — a powdered chalk — mixed with water and brushed onto windows to reflect sunlight and reduce indoor heat without electricity. The chalk's main ingredient, calcium carbonate, is effective at reflecting visible, ultraviolet and near-infrared light and is also studied for engineered cooling coatings. While inexpensive and immediately useful, the film is temporary and not a substitute for better insulation, reflective roofs or energy-efficient design. Other low-cost measures like reflective blinds, shading and sealing leaks can complement the approach.
French Residents Coat Windows With Blanc de Meudon Chalk to Beat Intense Heat — A Cheap, Temporary Cooling Fix
Photo Credit: iStock
As yet another intense summer heat wave spreads across France, many residents are turning to a low-cost, do-it-yourself solution: brushing a thin paste of Blanc de Meudon — a fine powdered chalk — mixed with water onto their windows. The pale, matt coating reflects sunlight away from glass, reducing the amount of solar heat that enters rooms without using electricity.
Retailers have reported surging demand for Blanc de Meudon, with some shops briefly running low on stock. When applied with a brush, the mixture leaves a light, opaque film that bounces incoming light back outside rather than allowing it to warm interior spaces.
"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," said Xiangyu Li, a cooling-technology researcher at Purdue University, in comments reported by the BBC.
Calcium carbonate is already being explored in engineered radiative-cooling coatings because of those reflective properties. Researchers have developed ultra-white paints that can reflect an extremely high share of sunlight (Purdue researchers reported one formulation that reflects up to 98.1% of incoming solar radiation), and cities are experimenting with cool roofs — reflective roofing surfaces designed to lower temperatures at scale.
What Users Can Expect
The chalk film is attractive because it requires no electricity: once applied, it passively reduces solar heat gain until it is washed away or removed. That can help households cut back on air-conditioning use and lower electricity bills, or provide modest relief where air conditioning isn't available.
Evidence and Related Experiments
Field and modeling studies suggest reflective surfaces can have measurable effects. For example, one modeling study estimated that broad adoption of cool roofs across London during a heat wave could lower average temperatures by about 1.4°F (0.8°C) and potentially avert hundreds of heat-related deaths. Smaller-scale experiments have produced interesting results too: researchers in the UK reported that a thin yogurt coating on windows reduced indoor temperatures by an average of 1.1°F (0.6°C), with drops up to 6.3°F (3.5°C) under very sunny conditions.
Limitations and Practical Considerations
Experts caution that chalking windows is a temporary, low-cost mitigation—not a substitute for proper building measures such as improved insulation, energy-efficient glazing, reflective roofing, or good urban planning. Practical downsides include reduced natural light and outward visibility, the need for reapplication after cleaning or rain, and the potential for residue or streaking on glass that requires careful cleaning.
Some specialty window coatings (for example, low-emissivity or certain factory-applied films) may be sensitive to abrasive cleaning or repeated applications of powders and solvents. Before widespread use on expensive windows, test the mixture on a small area and consult the window manufacturer's care guidance.
Other Low-Cost Options
Alongside chalk coatings, households can try reflective blinds or shades, external shading (awnings, shutters), sealing drafts and leaks, and applying cool-roof materials where feasible. Combined strategies often deliver the best comfort and energy savings.
Bottom line: Blanc de Meudon offers a cheap, readily available way to reduce solar heat gain through windows during heat waves. It’s a practical short-term tactic for lowering indoor temperatures and electricity use, but it should be used with awareness of trade-offs and as part of a broader approach to keeping buildings cool.
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