In Paris, where June 2026 has shattered temperature records with an average of 22.7°C, nearly 4 degrees above normal, residents seeking relief have turned to a striking solution: covering their windows with aluminum foil and thermal blankets. The sight has become common across the city, but architect Vincent Parasie, who leads Atelier Parasie Arquitectes SLP in Barcelona, warns that this approach reflects urgency, not a lasting answer to the problem.
Parasie describes the aluminum foil method as a stopgap: cheap and quick, but ultimately superficial. Applied to the outside of glass, foil can reflect some solar radiation before it enters the home, modestly reducing indoor temperatures in rooms most exposed to the sun. It cannot replace genuine building improvements. Foil is not designed for outdoor use, so it can peel away or degrade quickly, especially in harsh weather. Purpose-designed exterior shading blocks sunlight before it reaches and heats the glazing. Options include shutters and overhangs, while screens and louvers can also shade windows. Shade trees provide another option. Performance depends on the window’s orientation and local sun angles, with the geometry of the shade also playing a part.
There are trade-offs in comfort, too. Blocking windows with foil or thermal blankets cuts natural light and obscures views, both impressive to well-being at home. Parasie says the method may provide short-term relief, but it does nothing to address the underlying vulnerability of many homes to extreme heat. Window performance is often assessed using the solar heat-gain coefficient (SHGC), which measures how much incoming solar energy passes through, and visible transmittance (VT), which indicates how much daylight gets through. A lower SHGC can help limit overheating on sun-exposed façades, while a higher VT helps preserve daylight. The right balance depends on orientation and climate, as well as room use and the value of winter sun.
Beyond Quick Fixes
Parasie says the real issue lies in how buildings are constructed and insulated. The thermal envelope, comprising the facades and roof along with window systems, determines how quickly a home heats up and how well it retains cool air. Upgrading old windows to models with thermal breaks and double glazing, paired with solar control coatings, can significantly reduce heat gain without sacrificing daylight. These improvements filter out much of the sun’s energy before it can raise indoor temperatures. A 2026 review in the journal Buildings describes modeled cases in which triple glazing with low-g-value coatings reduced solar gains by 96.5%; another low-emissivity triple-glazing configuration cut solar gains by about 85.5% and sensible cooling loads by 52%. Those figures come from specific modeled configurations, not a guarantee for every home.
Traditional exterior shading remains highly effective when installed outside the glass. Shutters and screens are two options; louvers and brise-soleil can also shade windows. The logic is straightforward: stopping the sun before it penetrates the building is more efficient than cooling a room after it has heated up. Interior blinds can reduce glare and block some light, but they intercept sunlight after it has passed through the glass. Exterior shades keep more of the radiation from reaching the window in the first place. That can matter especially for large south- or west-facing windows exposed to intense afternoon sun. Parasie notes that these measures, sometimes overlooked, are proven to work in Mediterranean climates and beyond.
Shading design matters as much as the choice of device. A louver’s blade angle affects how much direct sun reaches the glass, as do its spacing and length. Its orientation also matters. A 2026 study published by Taylor & Francis reported indoor-temperature reductions of up to 4.0°C in Chabahar and 3.2°C in Chittagong for vertical louvers. Among the configurations tested, a 60-degree blade angle, 0.1-metre spacing and 0.4-metre blade length performed best across the study’s case-study cities. These findings show how design geometry affects results, not a universal specification for every building.
Window area also affects the balance between heat, daylight and seasonal comfort. The Buildings review reports that, in one modeled case, reducing the window-to-wall ratio from 55.3% to 25.2% lowered overheating failures from 214 to 38, while adding external shading eliminated the modeled failures. The review also cautions that excessive solar control can reduce useful winter heat gains and daylight. In practice, glazing and exterior shading should suit the site and façade, rather than covering every window indiscriminately.
Options for Renters and Non-Renovators
For those who rent or cannot undertake major renovations, Parasie recommends maximizing natural ventilation before relying solely on air conditioning. The most effective strategy is to ventilate at night, when outdoor temperatures drop. Opening a single window may not be enough: create a cross-breeze by opening windows on opposite sides of the home, or use a fan to move stagnant air. Either can make a noticeable difference.
The widespread use of aluminum foil in Paris highlights a growing reality: many urban homes are not built to withstand the intensity of modern heatwaves. Foil may offer brief respite, but it cannot replace proper insulation or exterior sun protection. Well-designed windows also matter. As climate extremes become more common, the need for lasting solutions is becoming clearer.
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