A recent analysis by the Argile.ai platform, which examined around 700,000 home energy audits published by Ademe, reveals that nearly one in three audits recommends improvements that can actually raise indoor temperatures during the summer. In 26.5% of the cases, these measures increase the internal temperature by 1 to 5 degrees Celsius, while in 4% of scenarios, the recommended works cause an increase of more than 5 degrees. This finding highlights a growing concern about how energy efficiency upgrades, intended to improve comfort and reduce energy use, can sometimes have unintended consequences during heatwaves. Pierre-Louis Guhur, co-founder of Argile.ai, explains that the most problematic combination is internal wall insulation paired with floor insulation, which can significantly raise indoor temperatures during extreme heat. He notes that this combination is particularly problematic during heatwaves, affecting more than eight out of ten cases. Low floor insulation placed directly on the ground slab, Guhur says, "cuts the house off from its only free 'cold well,' the ground," thereby reducing the home's natural ability to cool itself. As a result, 61% of projects that include this type of insulation worsen summer comfort. In contrast, external wall insulation, though more complex to install, can lower indoor temperatures by about 1.1 degrees Celsius. An example from a small 68-square-meter house in Vernouillet, Yvelines, illustrates the issue. During the summer 2026 heatwave, the indoor temperature without any recommended improvements would peak just over 35 degrees Celsius. However, with the energy audit recommendations from 2024—assuming windows are closed and shutters remain open—the temperature could exceed 44 degrees. This problem is more pronounced in northern regions, where the type of building and its original design play a significant role. Stone houses with thick walls on bare slabs, which naturally cool down, are the most affected by poorly calibrated insulation measures. Certain areas, such as the Landes, Gironde, Pyrénées-Orientales, and the Gers in the Southwest, are less impacted, with only 15% of recommended works leading to increased indoor temperatures during the hottest periods. In contrast, regions like Paris, the Hauts-de-Seine, and the Val-de-Marne have significantly higher rates of discomfort, with 53%, 48%, and 45% of recommended renovations worsening summer conditions, respectively. Guhur emphasizes that the problem lies not with insulation itself, but with the placement. He notes that external insulation is the only type that improves comfort in both winter and summer. However, local regulations, architectural guidelines, and co-ownership rules can limit homeowners' options. The current Energy Performance Diagnostic (DPE) includes a summer comfort indicator, but it is considered insufficiently detailed. It evaluates factors like building inertia, solar protection, cross-ventilation, and roof insulation, but does not consider the type of insulation or floor construction. Despite improvements in the DPE’s summer comfort rating, the proportion of projects that worsen comfort remains unchanged. Following the 2026 heatwave, the French government announced plans to reform the DPE, aiming to include more specific indicators for summer comfort. Whether this reform will effectively address the issue remains to be seen.