Thermal kettles: a study reveals that one in two homes is affected

Heat waves are becoming more frequent, and the record temperatures observed in recent years have put summer comfort back at the forefront of concerns for building professionals. Long focused on winter performance, energy renovation must now incorporate the ability of homes to remain habitable during periods of intense heat. It is in this context that IGNES and Pouget Consultants analyzed nearly nine million Energy Performance Certificates (EPCs).

While the fight against energy-inefficient buildings has largely guided public policy in recent years, summer comfort has long been a secondary concern. Yet, a home that performs well in winter is not necessarily able to remain comfortable during periods of intense heat. The absence of solar shading, insufficient ventilation, or a poor consideration of thermal inertia can transform a home with a good energy performance certificate (EPC) rating into a veritable "heat kettle."

The absence or inadequacy of external sun protection, such as shutters or blinds, appears to be the main factor in overheating of homes during periods of intense heat.

Faced with increasingly early and intense heatwaves, professionals in the sector believe that adapting housing for summer comfort is still progressing too slowly. Despite several warnings in recent years, they are calling on public authorities to quickly initiate discussions to accelerate the implementation of concrete solutions.

Summer comfort: even the most energy-efficient homes are affected

The study reveals that only one in ten homes offers a satisfactory level of summer comfort according to the dedicated indicator of the energy performance diagnosis (DPE). Conversely, nearly half of all homes are considered "heat kettles," a phenomenon mainly attributed to the absence or inadequacy of external solar shading.

One of the main findings of this analysis is that the most energy-efficient homes are not necessarily the best protected against extreme heat. In fact, a third of homes rated A or B on the Energy Performance Certificate (EPC) also have insufficient summer comfort and can be described as "heat pumps."

The study also highlights significant geographical disparities. Areas highly exposed to urban heat islands have a particularly high proportion of affected homes. This is especially true in cities like Paris and Lille, where more than half of all homes are likely to become "heat kettles" during heat waves.

Summer comfort must become a real focus of renovation and construction.

The study's results show that summer comfort is still insufficiently considered in both renovation projects and new construction. In energy audits, more than half of the reports do not recommend the installation of solar shading devices, even though the dwelling lacks them or is inadequately equipped. This finding illustrates the gap between the challenges posed by extreme heat and the solutions actually recommended.

The situation is similar for new housing. Despite the implementation of the RE2020 regulation, only 19% of newly built homes fully meet the criteria for good summer comfort. More broadly, the proportion of homes offering a satisfactory level of comfort is increasing only slightly with regulatory changes, raising questions about the effectiveness of current measures in the face of increasingly frequent heat waves.

The study also highlights several limitations of the "summer comfort" indicator in the Energy Performance Certificate (EPC). Nearly a third of the assessments do not include this indicator, while calculation errors persist. The authors also believe that the methodology could be improved to better account for key parameters such as the location of the dwelling, exposure to urban heat islands, the performance of solar shading devices, their level of automation, and the characteristics of the materials and insulation.

For Anne-Sophie PERRISSIN-FABERT, General Delegate of IGNES: “Despite repeated warnings in recent years from associations, elected officials, and professionals, housing adaptation is not progressing. The measures of the PNACC are still pending, and the lack of provisions in the draft housing stimulus bill was surprising. Rapid solutions can be found; we call for stakeholders to be brought together to finally take action.”

This study confirms that adapting buildings to extreme heat is becoming a major challenge for the entire sector. Solar shading, natural ventilation, material selection, insulation, automated blinds and shades, and green roofs are now complementary levers for energy efficiency. As heat waves become more frequent, summer comfort is expected to play an increasingly important role in both new construction and renovation projects.

While "energy sieves" are now well identified, "thermal kettles" illustrate another reality: a home that performs well in winter is not necessarily comfortable in summer.

About the study

The study consists of a statistical analysis of the ADEME DPE database (version of 05/01/2026), particularly the summer comfort indicator. In case of errors, it also corrects the indicators based on the data entered in the DPEs (Energy Performance Certificates), applying the calculation rules established by the decree of March 31, 2021, concerning energy performance diagnostics for buildings or parts of buildings used for residential purposes in metropolitan France. The study also analyzes the influence of other DPE parameters on the indicator value. Existing and new housing are considered, but not commercial buildings. Finally, it analyzes the regulatory energy audits contained in the ADEME Audit database.

[1] The results have not been adjusted and are therefore not representative of the French housing stock.

Illustrative image of the article via Depositphotos.com.