In Rennes, an eco-district at the cutting edge against heat domes

A recently emerged seven-story building built around a decades-old tree. In front, a wooded area and vegetated ditches bordering a car-free street: welcome to La Courrouze, a new 110 ha district in the south of Rennes.

"We are in a district of Rennes where, in heat waves or times of high temperatures, we have islands of coolness which are formed because the presence of plants is strong" explains Marc Hervé, first deputy for town planning at the City of Rennes.

At La Courrouze, no asphalt on the road. At the foot of a building, the city councilor shows cobblestones in pastel tones which form a draining pavement, to filter the water and nourish the vegetation.

On the roofs, solar panels. Further on, a public school on two floors, semi-open, which illustrates these new, more compact, airy constructions.

Metamorphosed, the former military district of the arsenal has here left a legacy of 40 hectares of vegetation to the Breton capital, which is experimenting with a new way of thinking about housing in the light of global warming.

"Ecological height"

Assisted housing, offices, shops, park and ride and carpooling area, before the arrival of the metro in September: eventually, this showcase eco-district initiated in the 2000s will house 10.000 inhabitants and 8.000 jobs.

Operations destined to multiply if the city wants to keep its commitment to allow "90% of Rennais to have a park 5 minutes from their home" by 2030 and break "the heat dome" as evidenced by the differences in temperatures. On July 18, a new record was set, with a 9°C difference between Rennes (23°C) and its countryside (14°C).

The metropolis, which adopted its climate plan in 2019, hopes to make this new green lung a cornerstone of its local intermunicipal urban plan (PLUi).

But in return, the city, which is experiencing a strong real estate tension, assumes its choice to build in height.

"It's not contradictory," says Marc Hervé. With a smaller footprint "the height is ecological", argues the elected official who emphasizes that the district devotes large spaces between buildings which can reach 11 floors.

The question of water

How to reconcile in the cities of the west, Rennes, Nantes or Angers, demographic growth and economic development without making the city denser and therefore hotter?

"This equation is not simple", answers Vincent Dubreuil, professor of geography at the University of Rennes 2 who has been studying climate change in France for 30 years.

"There are policies of disartificialization, of establishment of vegetation", but that "raises the question of water, because to be effective against heat islands, vegetation must consume water".

While Rennes - classified as a temperate climate, with cool summers and a humid climate - risks "switching more frequently to a Mediterranean class", choosing vegetation and trees with a high cooling index but also more water-intensive, n is not necessarily relevant, especially since the Rennes basin is one of those regions that is not very wet, observes the researcher.

According to Vincent Dubreuil, "we must now think about development solutions in terms of the nature of the materials, the colors of the buildings and the vegetation. And take into account the fact that, in any case, the climate will be warmer in the future".