One of the effects of global warming is an increase in heat waves, their duration, and their intensity. France is particularly affected by these extreme temperatures, mainly due to the density of buildings, impermeable soils, and the lack of vegetation in certain regions. So, what factors influence the summer comfort of a building, and how does thermal inertia play a key role in this?
INCREASE IN HEAT WAVES IN FRANCE
According to the 2022 heatwave and health report published by Santé Publique France, there were 33 days of heatwaves between June and September 2022. These heat waves affected 78% of the metropolitan population. They led to more than 200,000 heat wave-related medical visits (emergency room visits and SOS doctor consultations for hyperthermia, dehydration, and hyponatremia). This is two to three times more than during periods without heat waves.
33 days
heat wave
78 %
of the metropolitan population is affected
+200 000
use of heatwave-related healthcare services
How to protect yourself at home,
Extreme outdoor temperatures?
BUILDING AND SUMMER COMFORT
The RE2020 Environmental Regulation, which came into force on January 1, 2023, reinforces the concept of summer comfort in building design. Factors that have a direct influence on summer comfort include:
- the ambient air temperature,
- air speed,
- wall thickness,
- the orientation of the building,
- the characteristics of the materials,
- human metabolism,
- dress code,
- thermal tolerance, etc.
RE2020 also incorporates bioclimatic building design rules, including:
- anticipate the position and shape of the building in relation to sunlight,
- build heavy, thick walls to promote thermal inertia,
- insulate the walls, roof, and attic,
- limit thermal bridges,
- anticipate the orientation and glazed surface,
- create air movement to cool walls, etc.
When renovating buildings, these rules and factors could not always be taken into account during their design. Their users may therefore suffer from the effects of heat more frequently.
THE IMPACT OF THERMAL INERTIA
In both construction and renovation, slowing down the entry of heat into a building requires, among other things, dense walls that provide high inertia.
Thermal inertia is the ability of a wall to store heat and release it gradually. It plays a key role in summer comfort, as it helps to mitigate temperature variations indoors and maintain a stable temperature.
The walls
In France, walls are generally made of very dense materials, such as cinder block, brick, or concrete. Since these materials already have high thermal inertia, the internal temperature of the walls is generally close to the internal ambient temperature.
Roofs
Conversely, French roofs are made of dense but thin materials: slate or tiles for the roof covering and plasterboard as the final shield on the ceiling.
While the color of the roof covering plays a significant role, the lack of inertia of roofs in France has a direct impact on the temperature inside the attic, which can reach twice the outside temperature.
For example, if it is 30°C outside, the temperature inside the attic can reach 60°C.
This heat radiates to the ceiling and increases its temperature.
THE TEMPERATURE AS IT FEELS
The perceived temperature plays a major role in the concept of summer comfort. It is the average between the temperature of the inner surface of the walls and the ambient temperature of the room.
For example, if the ambient temperature of a room is 19°C and the average temperature of each surface (ceiling, walls, floors) is 21°C, the perceived temperature will be 20°C.
To ensure comfort in summer, it is therefore essential to maintain a low temperature on all surfaces (ceilings, walls, floors).
THE ROLE OF INSULATION IN UNUSED ATTIC SPACE
As we have seen, in the case of most French roofs, the roofing provides little or no thermal inertia due to the materials used (tiles, slates). Heat therefore penetrates easily into the attic space and radiates towards the ceilings, directly increasing their temperature and therefore the perceived temperature.
In this configuration, only the insulation can provide thermal inertia and thus protect occupants from heat radiation from the ceiling.
It is therefore essential to choose a thermal insulator with high inertia, so that heat from the attic does not enter the rooms.
cellulose insulation example, slows down heat transfer by up to 11 hours. This high phase shift allows for nighttime ventilation of the building when outside temperatures are lower, and reverses heat exchange.
In fact, if the peak heat is reached at 1 p.m., it will only enter the rooms of the house at midnight. At this time, the outside temperature has dropped and is therefore cooler. Residents can then open their windows to reverse the heat exchange: the cool outside air will enter the rooms while the heat will escape through the windows.
Conversely, with insulation that has low thermal inertia, heat quickly penetrates inside buildings. For example, mineral wool blown into attic spaces has a phase shift of approximately 4.5 hours. In our example, the peak heat reached at 1 p.m. will enter the rooms of the house at 5:30 p.m. At this time, the outside temperatures have not yet dropped, and the temperature felt inside will therefore continue to rise.
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