Heat Wave: Condo Owners Discover Their Buildings Have Turned Into Boiling Pots
Following the most intense heat wave ever recorded in France in late June—with nighttime temperatures reaching 29.5°C in Paris—the nature of apartment building renovations is changing. The issue is no longer just heating bills, but whether older buildings can remain livable in the summer.
Between June 24 and 28, 2026, France experienced the most intense heat wave on record since weather measurements began. Saintes reached 43.8°C, Saumur 44.1°C, and Paris exceeded 40°C for two consecutive days. But it was the night that shifted the perception of the problem: in the capital, the temperature did not drop below 29.5°C on the night of June 26—an unprecedented level at this latitude.
After several days of heat buildup, facades, walls, roofs, and asphalt continuously radiate heat, and indoor temperatures no longer drop to a tolerable level. The building itself becomes a source of heat—a thermal boiler day and night—with interior temperatures consistently above 30°C.
A housing stock designed for a climate that no longer exists
The problem is primarily generational. According to INSEE, one in five homes in France—nearly 7.5 million units—was built between 1946 and 1970, before any thermal regulations were in place. Condominiums built between the 1950s and 1990s were designed to retain heat in the winter, in the wake of the oil crises, but never to dissipate it in the summer.
Summer comfort was not included in regulations until 2005, and it was not until RE2020—which takes effect in 2022—that it became a criterion in its own right, measured by an “uncomfortable degree-hours” indicator that accounts for the duration and intensity of overheating.
When applied to older buildings, this framework reveals just how wide the gap is compared to housing designed for an entirely different purpose. “We are gradually seeing a shift from energy-efficient renovation toward true climate-responsive renovation. This means considering winter comfort, summer comfort, resilience, and energy efficiency all at the same time,” notes Sylvain Lefevre, president of Synergiec.
Passive Air Conditioning: A Multi-Million-Euro Project
The example of a 182-unit condominium complex in Bordeaux, supported by the firm, illustrates the scale of the work. Rather than installing air conditioning throughout the building, the co-owners voted for a passive cooling program: replacement of all windows and doors with the installation of sunshades for nearly 700,000 euros, exterior wall insulation for 1.7 million, insulation of the flat roofs for 780,000 euros to integrate the building’s mass into the insulated volume and ensure thermal phase shifting, replacement of a curtain wall for 180,000 euros, and renovation of the planters for 445,000 euros, which helps create a cooling island. Most of these items are eligible for existing programs, such as MaPrimeRénov’ Copropriété, energy savings certificates, the collective eco-PTZ, and local grants; work not yet classified as energy-related, such as the planters, can be covered by collective loans. With heat waves occurring earlier, more frequently, and with greater intensity, the issue of summer comfort will take center stage at upcoming condominium general meetings, on par with the Energy Performance Certificate (DPE).
