How Paris' historic architecture creates a boiling urban heat island

With extreme heat a top concern for the Paris Olympics, the City of Light's historic architecture has become a point of friction. According to Roofscapes Studio, a startup founded by three MIT graduates, up to 80% of Paris' roofs are made of zinc, a material that absorbs and radiates heat, exacerbating the urban heat island effect.

Why it matters: This makes Paris extremely vulnerable to heat waves. Last summer, four heat waves hit France, causing more than 5,000 deaths.

• A high density of buildings and low green space means heat is easily trapped and slow to dissipate, often making the city hotter than its European counterparts.

Driving the news: Paris is hosting the 2024 Summer Olympics, which will bring an influx of visitors to an already vulnerable region.

By the numbers: Temperature measurements show that the air one meter above a zinc roof can peak at more than 131° Fahrenheit, nearly 30° F greater than the Paris air temperature on the hottest days.

• The effect also boosts temperatures on the top floors of zinc-roofed buildings.

What they're doing: Roofscapes Studio installs wooden platforms on top of the zinc roofs to reduce the worst of the heat.

• Since wood can still radiate a certain amount of heat, they are developing a "green roof" to include planted vegetation, which can create energy and remove heat from the air.
• A decommissioned city hall which was turned into L'Académie du Climat (the Climate Academy) had a green roof installed through a pilot program with Roofscapes Studio, and it hopes to expand this program throughout the city.
• The green roof module has shown a 10° Celsius difference (18° F) between the air outside and the temperature inside the top floor of the building at 2pm on a hot day.

What they're saying: Olivier Faber, a founder of Roofscapes Studio, told Axios, "We wanted to work on the roofs because they're more than a third of the city's horizontal surface in dense urban centers like Paris." He explains that roofs are a societal opportunity for climate adaptation, as they typically sit empty and have a huge surface area.

• Faber continues, "[I]f the zinc is protected from solar radiation, its surface temperature will never be above the temperature of the air, which in a heat wave could be 35°-40° C (95° to 104° F) maybe. But it will never be 75° C or 80° C (167° to 176° F)," so the first step is to shade the roof.

What's next: The city of Paris plans to implement green roofs to keep the city cooler during warmer months, but approvals and installations will be an uphill battle.

• Because of the historic nature of Paris' buildings, builders require permission from a preservation architect for any modifications.