Under the zinc roof, the furnace: in Paris, the puzzle of cooling the roofs

Survival blankets on the windows, fans, nighttime ventilation...: Ana Puhac Crouzet is doing everything she can to prevent her 37 m2 apartment, on the 5th floor of a building in the 19th arrondissement, from turning into a kettle.

"It was a thermal sieve when we bought it. The attic insulation work worked for the winter... not for the summer," sighs this pregnant woman, who is dreading the next heat wave for her future baby.

In summer, zinc, which covers nearly 80% of Paris's 128.000 roofs, can reach temperatures of up to 80°C in the sun, according to the Ile-de-France Regional Directorate for the Environment, Planning and Transport (Drieat). The heat spreads into the attics, turning homes into ovens.

The use of this ultra-light metal made it possible, from the 1850s onwards, to create thousands of mansard-roofed apartments to house the servants of wealthy families on the lower floors, in a capital that was already "overcrowded".

"There wasn't really a rich or poor neighborhood, segregation was mainly vertical," Julien Bigorgne, an environmental engineer at the Parisian Urban Planning Workshop (Apur), told AFP.

"The Aristocats"

In the 19th century, people in Paris died of cold, primarily in attics, the expert points out.

The problem has reversed with global warming, as shown by the deadly heat wave of 2003, where elderly people living on the top floors found themselves "trapped", pointed out Karine Laaidi of Public Health France, during a "Living Well Under the Roofs" symposium in May.

It is therefore urgent to adapt this "fifth façade", as the capital could experience heat peaks of 50°C by 2050. The possibilities remain limited however in a city where 94% of the buildings are subject to heritage protection.

"It's so iconic, the setting for so many films, The Aristocats... the debate is passionate, so when you live on the top floor your voice is not heard," Alexandre Florentin, who has just created "The Top Floor Collective," told AFP.

For this former Paris councillor, who chaired the "Paris at 50 degrees" mission, "the status quo is no longer possible." "Given the speed of climate change, we don't see how we could afford the luxury of not resorting to solutions that change the aesthetics a little," he believes.

"Sarking" as a barrier

Like "sarking", an increasingly common technique consisting of raising the roof by removing the existing covering, adding a layer of insulation to avoid direct contact with the zinc, then installing a new one.

"From a thermal point of view, it is currently the most effective method," explains Charles Lemonnier of the Paris Climate Agency (APC). It also has "the advantage of renewing the zinc for 50 years and preserving the know-how of the zinc workers," which is listed by UNESCO as intangible cultural heritage of humanity.

The technique does not alter the landscape and is therefore "quite generalizable" according to him for sloping roofs - unlike green roofs, which are suitable for flat surfaces.

"We've lost 4 degrees!", rejoice Alain and Martine Tousla, an octogenarian couple who have lived in the attic near République since 1986 and whose summers had become "unbearable".

"Sarking gives the best performance but don't be fooled: above 40 degrees, you would need a 50 cm layer of insulation... which makes the roof swell and swallows the skylights," Bruno Sarre, a roofer and zinc worker, told AFP.

"Beyond 30 cm, it's difficult to integrate aesthetically," especially on the street side, adds Marie-Jeanne Jouveau, architect and heritage urban planner. For her, air conditioning will become "inevitable" in certain situations.

"There is a risk that serious health problems will occur before adaptation solutions are implemented. So the question of cooling will inevitably arise," anticipates Julien Bigorgne.

But "we don't have the means to finance individual air conditioning units with the energy crisis," warns Maud Lelièvre, rapporteur for the "Paris at 50 degrees" mission. She compares the situation under the roofs to areas rendered uninhabitable by coastal erosion, and mentions "natural disaster measures" to evacuate attic dwellings.