DPE ratings: why home energy tests may miss summer heat risks

"I have received many comments from visitors praising how cool the place is,” says Gilles Alglave, owner of the Prieuré de Saint-Arnoult (Oise), a 15th-Century priory that he opened to visitors over the summer.

Visitors sought refuge in the priory, whose timber-framed architecture naturally insulates its interior and kept it cool during the scorching temperatures affecting the Oise.

Across France, tourists flocked to museums, churches and cinemas in search of cool spaces and air conditioning. Owners of old houses, often built from thick timber or stone, emerged as unexpected beneficiaries of the summer heatwaves. At the same time, the term bouilloire thermique – thermal kettle – has gained currency to describe a home that traps heat.

Worst affected were residents of top-floor flats, urban buildings and poorly insulated homes, where temperatures sometimes became unbearable. The problem was particularly severe for Mélanie Gassen, a pregnant cashier living in a top-floor flat in Metz (Moselle), whose experience was documented by local newspaper L’Est Républicain. She slept in her building’s corridor in search of cooler air because her 20m² studio trapped so much heat that temperatures reached 35°C to 40°C during the day and remained between 30°C and 32°C overnight.

Her front door remained open, while she kept her television switched off and avoided charging her phone to prevent the temperature rising further. She slept for no more than two hours at a time because of the heat.

The rise of bouilloires thermiques has fuelled debate over whether France’s Diagnostic de performance énergétique (DPE) should be reassessed. It rates the energy consumption and efficiency of buildings on a scale from A to G, with A the most efficient and G the least.

The DPE was designed largely to tackle the opposite problem: passoires thermiques, or thermal sieves – homes that are unable to retain heat during winter. The government encourages owners to carry out insulation work through financial incentives such as MaPrimeRénov’, with the aim of improving homes rated F and G to grades C, B or A.The lower the rating, the greater the disadvantages for owners.

Property values can fall, and the least efficient homes are gradually being barred from the rental market. G-rated properties have been affected since January 1, 2025, with F-rated homes due to follow on January 1, 2028, and E-rated homes on January 1, 2034.

Among those who feel disadvantaged by the system are owners of a bâti ancien – buildings constructed before 1948 – which are frequently given E, F or G ratings because of their poor winter energy performance.

However, many owners who insulated their homes assumed that achieving an A, B or C rating would also protect them from extreme summer heat. Some instead experienced a double-edged effect, with newly insulated buildings retaining unwanted heat.

The Prieuré de Saint-Arnoult, a bâti ancien, did not. “Thermal insulation alone is not an accurate measure for old buildings,” said Clément Gaillard, founder of design consultancy Freio.

Mr Gaillard said older structures, although often labelled passoires thermiques, were historically uncomfortable in winter but relatively pleasant in summer. Their natural thermal inertia, combined with materials such as stone and thick timber, can regulate indoor temperatures effectively.

The DPE gives limited weight to this characteristic, known as confort d’été – a building’s ability to maintain a pleasant indoor temperature during hot weather without relying on active cooling systems such as air conditioning.

“When comfort enters the equation, the DPE is an unsuitable assessment,” said Mr Alglave, who has been a member of Maisons paysannes de France, an association dedicated to protecting and promoting traditional rural buildings, for 50 years and is now its president. The association questioned the accuracy of the DPE’s calculation method. In 2011, one of its projects showed that bâti ancien buildings were graded C or D and not E, F or G, or passoires thermiques.

Housing and Urban Renovation Minister Vincent Jeanbrun announced that the government would give greater consideration to confort d’été when calculating DPE ratings. For now, summer comfort is assessed using three smiley faces – sad, neutral and happy – based on five criteria. The result has no effect on the final DPE grade.

Throughout the summer, shutter manufacturers emphasised the benefits of blinds and shutters in reducing indoor temperatures. Air conditioning and cooling-system companies also promoted their products as households sought relief from the intense heat.

Mr Gaillard advocates a bioclimatic approach to architecture, in which buildings are adapted to local conditions to regulate temperature. It challenges the idea that people can live anywhere in standardised homes while maintaining identical lifestyles, regardless of the climate.

Mr Alglave sees the renewed interest in traditional construction as a form of vindication.

“The intelligence of our ancestors and the pragmatic way in which they adapted to the climate have been neglected and disregarded,” he says. He linked this neglect to France’s centralised approach to policymaking, arguing that policies are too often designed for Paris and other large urban centres without sufficient consideration for regional conditions.

“Today, residents are treated as consumers of space. They are housed rather than being allowed to inhabit their homes,” he says.

“They have been stripped of both the act of living – understanding how to use a space – and the art of living: the search for comfort. As heatwaves repeat themselves, confort d’été will override confort d’hiver in the years to come.”