Heat wave: how to have cooler buildings?

How do you protect a building from heat?

There are two levers for action: "protect yourself from solar radiation, and be able to let the heat out", points out Karine Jan, head of the Sustainable Building department at Cerema (Centre for studies and expertise on risks, environment, mobility and planning).

For the second part, the most effective solution is the most obvious: airing at night. But it is from the design stage that this can be facilitated, with so-called through buildings, that is to say open on at least two facades, which make it possible to create air currents.

Inside, ceiling fans, already very present overseas, should also become more and more common, predicts Karine Jan.

To reduce exposure, it is necessary "to limit the large glazed surfaces open to the south", explains Loïs Moulas, director general of the Observatory of sustainable real estate, "introduce alcoves, terraces, to ensure that there are gray areas".

You can also add "sunshades" to the windows, wooden slats oriented in a way that allows light to enter in winter, when the sun is low, but not in summer, when it is high in the sky. heaven, he explains.

And of course the installation of shutters to be cool without being in the dark, as there are already in the South (spanners, Nice shutters...).

The most crucial objective is to avoid the installation of air conditioners, which aggravate global warming and increase, in a vicious circle, the need to use them, underlines Loïs Moulas.

Which materials are the most effective?

The heaviest building materials, such as cut stone or concrete, are the most insulating. On the other hand, they are also expensive and polluting.

They therefore conflict with the 2020 environmental regulations (RE2020), in force since January 1. To limit greenhouse gas emissions from new construction, it favors so-called biosourced materials (of natural origin), such as wood.

But with biosourced materials, we can combine effective insulators, such as concrete with wood... or biosourced insulators too, just as effective, such as hemp or wood wool, or cellulose wadding, says Karine Jan.

What is the State doing to adapt buildings?

The RE2020 makes summer comfort, ie protection against heat, an essential criterion for new buildings.

The indicator for measuring it is called degree-hours of discomfort. When the architect designs a building, the number of hours in the year (heat waves included) is calculated when the interior temperature should theoretically exceed a certain threshold, generally 26 degrees.

Each hour that this threshold is exceeded, one more degree represents a point of discomfort. And over a year, the building will not be able to accumulate more than 1.250. Beyond 350 degree-hours of discomfort, a building may be allowed, but it will be considered more energy-consuming, because the occupants will be all the more likely to install an air conditioner there.

To calculate it, we will take into account a multitude of criteria, explains Marc Schoeffter, engineer in the building department of Ademe (Agency for the environment and energy management).

"Summer comfort will be taken into account in relation to the size of the building, the size of the openings, the solar protections that we will be able to put there, the inertia... a building which has a strong inertia will be able to absorb the solar wave during the day and restore the stored energy during the night".

The energy performance diagnostics (DPE), mandatory for selling or renting a property, also take into account summer comfort parameters: insulation, presence or not of shutters, possibility of creating drafts, etc.