Building on the roof, a godsend for the thermal renovation of old buildings

Louis du Merle knows something about it. Having become a young owner moving to Issy-les-Moulineaux, near Paris, in 2019, he is impatiently awaiting the start of work in his poor-quality 70s building whose collective heating is oil-fired and fairly obsolete. Four floors today, eight tomorrow: the building permit was obtained in 2022.

"If it succeeds, we will demonstrate that even a copro not jojo can build an energy efficient building", he imagines. Windows, insulation from the outside, boiler room, entrance door, ventilation, everything will be renovated and an elevator installed.

"A great experimental adventure", he says, but also a cost of 421.000 euros for the condominium, entirely self-financed by the sale to a promoter of the right to build 500 m2 of new raised housing.

"This is a very interesting example where we are going to create six new housing units, while remaining on a small unit, this is called soft densification, and take a thermal sieve building to a C label. ", explains the architect Didier Mignery, whose agency UpFactor is supporting the project.

Contemporary touch

Some 5,2 million homes, or 17% of the stock, have a poor energy performance diagnosis F or G, on a scale ranging from A to G, according to the National Observatory for Energy Renovation.

"We can't raise everything, fortunately", adds Mr. Mignery, but "the potential is around 10% of existing buildings". It all depends on the local urban plan, the year and type of construction, seismic regulations and the possibility of finding developers who tend to come backwards, he explains.

"It won't solve the housing or energy renovation crisis, but it helps. It would make it possible to deal with 85% of housing construction needs for the next ten years, it's considerable and it's another way of thinking for the elect," he said.

Estimated between 125 and 300 billion euros, the energy rehabilitation of buildings is progressing too slowly in France, noted in November the Economic and Social Council (CESE), recommending the elevation among the solutions to be activated.

Aesthetically, adding floors can bring a contemporary touch or be invisible. Technically, it can even strengthen the building, assures the architect Jean-Thomas Finateu: "We remove the weight by removing the old roof, we build a new floor carrying the upper floor and we have a concrete chain around which the weight is distributed".

Martingale

Financially, it's the martingale.

"It's not new, it's part of the natural evolution of cities, but it helps raise money to renovate", underlines the architect François Pelegrin, who speaks of "hidden treasure" but warns: "C is very long and you need a lot of pugnacity".

His book published in 2021, "Happiness is on the roof", is full of examples of projects blocked by the appeal of a co-owner living on the top floor or by a town hall.

"It takes a lot of time and money, a lot of pedagogy in condominiums and there are regulatory brakes in terms of town planning", abounds Eli Ben Sadoun, commercial director of the specialized promoter L'Atelier parisien de elevation which delivered two achievements since its creation in 2015. The company claims around fifty projects in progress.

In the 19th arrondissement of Paris, it is completing the renovation of n°5 rue du Dr Potain. Three floors have been added to this 1910 building which had six, and six apartments put up for sale, at high prices for the area, more than 10.000 euros per m2 justified by a view of the Eiffel Tower.

Double-glazing, new boiler room, facades insulated from the outside over the entire height: rue Potain, the charm of the old has not completely disappeared, nor certain disadvantages such as the crampedness of the accommodation, but the building is offered a facelift, and even an elevator.

"It was a mega thermal sieve. We did all the work for zero euros, it would have been silly to say no!", Welcomes Patrice Petriarte, president of the union council.

Even if the construction site, started in 2020, is playing for extensions, the monthly heating savings are already monumental: "We went from 150 to 45 euros".