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

Paris – 09.07.2026: In France, during recurring heatwaves, reports are multiplying of homes that become much hotter inside than the outdoor air and are sometimes virtually uninhabitable. However, such cases of thermal boilers cannot yet be recorded unambiguously. Studies, administrative data and tenant reports point to a widespread problem, but the criteria for officially classifying a home as overheated are neither standardized nor generally available.

The core of the deficit is the data base. The summer comfort criterion introduced with the RE2020 building standards uses a degree-hour indicator that theoretically makes it possible to assess the overheating burden. However, such values rarely exist for the large stock of older buildings, and central registers are incomplete. Energy performance certificates (DPE) also contain heterogeneous data, have in many cases been prepared without systematic consideration of summer conditions, and allow only limited conclusions about heat peaks in everyday life.

In addition, there are methodological differences between analyses by consulting firms, research consortia and associations. Thresholds, assessment periods, simulation assumptions and climate data vary, as does the consideration of factors such as year of construction, top-floor location, orientation, shading, external shutters or the ventilation concept. Comparisons between data sets are therefore unreliable and lead to uncertain assessments of whether a home actually exceeds the thermal boiler threshold.

In practice, assessment is complicated because many households do not carry out continuous indoor temperature measurements. Spot measurements during a heat peak provide snapshots that are only partly convincing in court or for subsidy programs. Experts advocate standardized and reproducible measurement protocols, with clearly defined periods, documentation of ventilation behavior, sun protection and tropical nights, supplemented by secure data storage.

The consequences primarily affect vulnerable groups: older people, chronically ill people, young children and low-income households suffer particularly from persistent heat in poorly protected homes. Associations and municipalities are therefore pushing to strengthen the DPE, require mandatory information on summer comfort in listings, and specifically promote measures such as external sun protection, insulation, green roofs and effective nighttime ventilation. For new buildings, planners are calling for consistent application of summer comfort assessments; for existing buildings, practical measurement standards that make it possible to set renovation priorities fairly.

In the short term, experts recommend documenting indoor and outdoor temperatures over several days using simple data loggers, optimizing shading and cross-ventilation during the day, and using municipal advisory services. In the medium and long term, unified 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 criteria of a thermal boiler—and which measures are most effective in remedying it.