Boiler-fired housing alert: which departments will be most at risk in 2050?

The situation is alarming: more than 80% of French homes risk turning into "thermal kettles" due to the predicted rise in temperatures.

Since the 1990s, the average annual temperature in France has already increased by +1,7°C, and Météo France anticipates +2,7°C by 2050[1].

The most exposed departments – Vaucluse, Gard and Lot – would even see their number of very hot days double, reaching 12 days on average by 2050, according to forecasts from Météo France Climadiag.

However, the residential stock remains very poorly equipped: almost 80% of DPEs providing the "summer comfort" indicator are classified as "insufficient" or "average"; and more than one in three homes display an insufficient level[3].

In the areas most at risk from heat by 2050, three out of four homes already have an "insufficient" or "average" indicator, which places their occupants at the forefront of summer energy poverty[3].

Ranking of departments according to their risk index by department calculated by Hello Watt and corresponding to: current proportion of housing with “average” or “insufficient” summer comfort in the DPE × number of very hot days projected in 2050

Sources: Hello Watt calculation based on ADEME DPE data (July 2021 to October 2024) and 2050 climadiag Météo France forecast data for 32.000 municipalities in France.

A summer comfort indicator still insufficient to measure a major risk

To date, summer comfort remains a marginal indicator in the energy assessment of homes. While it is mentioned, it is not included in the calculation of the rating and remains a mere indication. This lack of consideration leads to its neglect in decisions regarding sales, rentals, or renovations.

Its reliability is also highly contested: the algorithms are incomplete, some housing units are not evaluated, and the methodology neglects essential parameters, such as location or thermal inertia. According to the Pouget Consultants-Ignes study (July 2024), a revision of the indicator would shift 90% of the housing stock to "insufficient" or "average."

This shortcoming is all the more problematic given that some homes, although well-rated by the DPE for winter, offer poor summer comfort. Buildings with high thermal insulation performance can thus turn into veritable "thermal boilers" in summer.

Summer comfort must become a pillar of energy assessment. Hello Watt makes several recommendations for both individuals and public authorities.

For individuals, 3 concrete levers:

• Insulate walls and roofs with bio-sourced materials with a high thermal phase shift - i.e., materials capable of delaying the penetration of heat into the wall. This work, if done correctly, remains the most effective solution. An unsuitable material can, in the middle of summer, transform a home into a furnace and prove more detrimental than the absence of insulation. For wall insulation, Hello Watt recommends wood wool, a bio-sourced material that delays the entry of heat by around eight hours, twice as long as expanded polystyrene. For the roof, we recommend Nita-Cotton insulation with a phase shift duration of 5 hours.
• Install exterior blinds on windows. A window equipped with shutters reduces solar gain by 85%. This is the simplest, least energy-intensive, and least expensive technique for lowering the interior temperature by 2 to 5°C. This is why Hello Watt systematically includes shutters in our window replacement offers.
• Use a reversible air conditioning system to cool your home in summer and heat it in winter; air-to-air heat pumps, also installed by Hello Watt, are ideal for this purpose. Their operation is simple: they capture external calories and release them via the indoor units in the form of heat or coolness, ensuring comfort and efficiency.

Hello Watt recommends several developments to existing systems to the public authorities:

• register the occultations and reflective coatings in MaPrimeRénov';
• increase support for high phase shift insulators;
• reform the DPE: make the summer comfort indicator more reliable, integrate it into the overall score and make it a mandatory information criterion.

For Pierre-François Morin, Director of the energy renovation activity at Hello Watt: "A home can display a B on the DPE and still become uninhabitable in the summer. This creates a real misunderstanding among individuals, who end up losing trust in the tool. It is urgent to make the DPE more readable, more reliable, and above all more consistent with current climate issues. But beyond the observation, there are already concrete solutions to improve summer comfort: insulating your roof, installing suitable shutters, reinforcing the inertia of the walls, or even opting for a reversible heat pump. These are simple, effective levers that we offer on a daily basis at Hello Watt. Summer comfort should no longer be a secondary criterion in energy renovation."

As the Ministry of Housing prepares to publish initial proposals this summer to update the DPE, Hello Watt is calling for an ambitious reform based on accurate data, genuine consideration of regional specificities, and increased support for households.

Adapting housing to extreme heat is no longer simply a matter of comfort, but a public health emergency and an energy imperative.

[1] Metéo France data - Climadiag projection 2050 for 32.000 communes in France.

[2] Pouget Consultants-Ignes study, published in July 2024.

[3] ADEME DPE data, July 2021 - October 2024.

Illustrative image of the article via Depositphotos.com.