At a time when heat waves are increasing, certain clues are clear: here is how to spot them to avoid buying a thermal kettle.
Regret: in recent weeks, this is the feeling that dominates among some real estate buyers. They discovered to their cost that housing with an excellent energy performance diagnosis (DPE) is not systematically designed to withstand extreme heat.
Nearly a third (31%) of homes classified DPE A are considered thermal kettles, revealed the consulting company Pouget Consultants, in August 2024. “Good energy performance within the meaning of the DPE does not necessarily guarantee a good level of summer comfort,” summarizes Pouget Consultants.
Faced with the increase in heat waves, what should we do? How can you recognize a thermal kettle if the energy class is not, in itself, a reliable indicator? What are the clues that allow you to identify at-risk housing during a visit or in a real estate ad?
The first thing to check is the summer comfort indicator. It appears in the energy performance diagnosis and notes, on three levels (good, average, insufficient), the ability of the home to remain cool without air conditioning. Its calculation adds several technical criteria: “presence of solar protection on the bays and on the roof, insulation of the roof, inertia of the building, crossing nature of the housing, presence of fixed air fans”, details the Ministry of Housing.
“Nevertheless, this indicator must be improved to better reflect realities,” admits the ministry. Massive calculation errors (26% of indicators turn out to be incorrect) and failure to take into account the urban heat island effect: the summer comfort indicator is not reliable on its own for assessing the real risk of overheating of a home during a heatwave. “The indicator is based on important elements but remains insufficient. It forgets to take into account the insulation of the roof, the walls, the orientation or even the floor of the accommodation,” regrets Damien Barbosa, coordinator of Rénovons, a collective which includes the Housing Foundation.
Beyond this indicator, “one of the first elements to check is the presence of shutters, if possible in wood,” says Damien Barbosa. The accommodation must also be through. The Environment Agency points out that even a well-insulated home must be able to evacuate the heat accumulated during the day; without nighttime cooling, it can produce a “thermos” effect. This is why windows located on opposite facades are ideal: they create air currents which cool not only the air, but also the walls, floors and ceilings (which will restore this freshness the next day).
The orientation of the windows of the house is also a crucial factor. Contrary to popular belief, due south is not the worst exposure, because in the middle of the day the sun is very high and hits the roof more than the windows. In reality, the most sensitive facades are oriented from southwest to northwest: at the end of the afternoon, the sun is lower and its rays strike the glazing almost horizontally, at the very time when the ambient air is already very hot. Naturally, location, being in an urban heat island and window size also play an important role.
But even if you are very careful, finding a thermal kettle is far from easy. “When possible, the best is to visit a home in real conditions, during a heat wave, in order to check how it resists high temperatures. This is what I did just yesterday,” underlines Damien Barbosa.
