Why it is difficult to clearly classify homes as “Bouilloire thermique”

Paris – July 9, 2026: In France, reports are increasing of homes in which indoor temperatures become significantly higher than outdoor air during recurring heatwaves, making them temporarily difficult to live in. However, these “thermal boiler” cases are not yet clearly documented. Research, administrative data, and tenant reports all suggest a widespread problem, but the criteria for officially classifying housing as overheated are neither standardized nor available nationwide.

The core of the problem lies in the data infrastructure. The summer comfort criterion introduced under the RE2020 building standard theoretically allows an assessment of overheating exposure using the degree-hours indicator. However, these values barely exist for most older existing buildings, and central registers contain many gaps. Energy performance certificates (DPE) also contain inconsistent information and are often prepared without a systematic summer assessment, limiting their ability to draw conclusions about sharp temperature rises during everyday heatwaves.

In addition, there are methodological differences among analyses by consulting firms, research alliances, and organizations. Thresholds, assessment periods, simulation conditions, and climate data vary, as does the treatment of factors such as building age, whether an apartment is on the top floor, orientation, shading, external shutters, and ventilation systems. As a result, comparisons between datasets are hardly reliable, making it difficult to obtain a dependable assessment of whether a home actually exceeds the “thermal boiler” threshold.

Practical assessment is further complicated by the fact that many households do not continuously measure indoor temperatures. Single measurements at the peak of a heatwave show only a momentary situation and are often not sufficiently persuasive in court cases or subsidy programs. Experts therefore call for standardized, reproducible measurement protocols with clear time periods, documentation of ventilation behavior, shading conditions, and tropical nights, as well as reliable data storage.

The effects particularly impact vulnerable groups: older people, people with chronic illnesses, young children, and low-income households suffer especially from prolonged high temperatures in insufficiently protected homes. Organizations and municipalities therefore call for a revision of the DPE, mandatory disclosure of summer comfort in property listings, and targeted support for external shading, insulation, green roofs, and effective nighttime ventilation. For new buildings, planners demand consistent application of proof of summer comfort; for the existing housing stock, practical measurement criteria are needed to determine renovation priorities fairly.

In the short term, experts recommend recording indoor and outdoor temperatures over several days with simple data loggers, optimizing shading and cross-ventilation during the day, and using municipal advisory services. In the medium and long term, standardized thresholds, a central data framework, and reliable funding channels are needed. Only then will it be possible to assess transparently, comparably, and with legal certainty whether a home meets the “thermal boiler” criteria and which measures will improve it most effectively.