Homes with good energy performance ratings but stifling temperatures during heatwaves: the shortcomings of summer comfort

Up to 36°C in a B-rated apartment

With outside temperatures reaching 40°C, residents are complaining about excessive heat in homes that have received good energy performance ratings (DPE). In most cases, the lack of shutters or sunshades is directly blamed.

"Our apartment is very well insulated; in winter we hardly ever turn on the heating." testifies Louis Legrand, a resident of Kremlin-Bicêtre, in the Val-de-Marne.

During the heat wave at the end of June, the temperature in his home rose to 36°C.

His building was completely renovated by a developer a few years ago and has a B rating on the DPE (Energy Performance Diagnosis). This system classifies buildings from A, for the most efficient, to G, for the least energy-efficient.

When Louis Legrand bought a well-rated property, he didn't expect to experience such temperatures in the summer. His case is far from unique.

According to a study carried out by Pouget Consultants and the Alliance of manufacturers of electrical and digital building solutions (Ignes), a third of homes classified A or B on the DPE would not be sufficiently adapted to heatwave episodes and could therefore be considered as "thermal kettles".

A summer comfort indicator separate from the EPC rating

The Energy Performance Certificate (EPC) does include an assessment of summer comfort, represented by a "smiley face". However, this indicator remains separate from the main rating, which is primarily based on energy consumption and greenhouse gas emissions.

The summer comfort assessment considers five parameters, including the presence of solar shading on windows, the installation of ceiling fans, and whether the dwelling is air-to-air. However, these criteria have no bearing on the letter assigned to the dwelling.

Tom Sarrebourse, an engineer at Pouget Consultants, explains that a dwelling is directly considered insufficient from the point of view of summer comfort when its glazed walls facing south, east and west do not have solar protection.

This is particularly true of Louis Legrand's apartment, which has no shutters. A dwelling located on the top floor, under an uninsulated roof, is also automatically considered inadequate.

Other criteria, such as the cross-ventilation of the dwelling, the presence of air circulators or the inertia of the building, then determine whether the indicator comes out as average or good, explains Tom Sarrebourse.

This methodology has the advantage of being simple. Christophe Rodriguez, director of the French Institute for Building Performance (Ifpeb), points out that approximately 70% of heat enters homes through solar radiation via windows. He also emphasizes that occupants of attic apartments suffered particularly during the 2003 heatwave.

The summer comfort indicator, however, remains far less visible than the energy performance certificate (EPC) rating. The Housing Foundation is therefore campaigning to make its display mandatory in every real estate advertisement.

The "thermos effect" of highly insulated homes

More broadly, building insulation, essential to combat energy-inefficient buildings, is sometimes implicated in the "thermos effect" described by occupants suffocating in their homes.

"Thermal regulations have a winter DNA and seek to retain heat from heating by over-insulating," Christophe Rodriguez points out.

"But if you have the misfortune to forget the sunshades or if you don't have the possibility of properly ventilating the dwelling, the heat gets in and it becomes very difficult to get it out," he continues.

Effective insulation can therefore retain heat inside when the dwelling lacks both effective solar protection and sufficient ventilation.

The choice of insulation and the thermal inertia of the building in question

The materials used also influence the thermal behavior of the building. According to the experts interviewed, wood wool, in particular, can slow down heat transmission for longer than glass wool or polystyrene.

Tom Sarrebourse also points out that interior insulation can reduce the thermal inertia of the walls. This inertia refers to the capacity of walls, floors, and ceilings to act as a buffer and limit temperature spikes within the dwelling.

Jean-Philippe Ndobo-Epoy, from the professional insulation association Mur Manteau, explains that this thermal inertia depends in particular on the mass of the walls. He cites the example of cathedrals, whose walls are thick enough to maintain a cool interior temperature for several days.

This phenomenon also helps to explain why some old houses with thick stone walls, sometimes poorly rated in the DPE, remain pleasant to live in during heat waves.

Insulation remains useful, provided that summer comfort is addressed.

The three experts interviewed nevertheless emphasize that insulation does contribute to improving summer comfort by limiting heat gain through the walls and roof.

However, its effectiveness depends on a comprehensive approach combining insulation, thermal inertia, solar protection, ventilation and air circulation.

"When you renovate a sieve, you're just one shutter and one air vent away from not turning it into a kettle either." Christophe Rodriguez sums it up.

Illustrative image of the article via Depositphotos.com.