The increase in heat waves in France is changing building performance requirements. Heat waves are now more frequent, longer, and more intense, with a direct impact on occupant comfort.
In this context, RE2020 places greater emphasis on addressing summer comfort from the design phase of buildings. In renovation projects, this issue is even more pronounced, as existing buildings were not designed to meet these climatic challenges.
For insulation professionals, this means understanding key concepts such as thermal mass, thermal lag, and perceived temperature.
Why buildings overheat in the summer
The Impact of Heat Waves
Heat waves affect a large part of the country and have significant health implications. They also highlight the limitations of traditional building systems, particularly when it comes to roofing.
The weak point of unused attic space
Attics can reach very high temperatures. When the outside temperature is 30°C, the temperature in an uninsulated attic can reach 60°C.
This phenomenon can be attributed to several factors:
- thin roofing materials, such as shingles or slate
- low thermal mass in the roof
- rapid heat buildup under the covers
The heat then radiates toward the ceiling, which directly raises the indoor temperature.
Thermal inertia: a key factor in summer comfort
Thermal inertia refers to the ability of a material or wall to store heat and release it slowly. It helps mitigate temperature fluctuations and maintain a more stable indoor climate.
It depends on several factors:
- specific heat capacity
- effusiveness
- diffusivity
In practice, it is measured using the thermal lag, which is the time it takes for heat to pass through a wall.
Difference between walls and the roof
In France, walls are typically made of dense materials, such as concrete or brick. They provide high thermal mass and help maintain a relatively stable indoor temperature.
Conversely, roofs are made of dense but relatively thin materials, which greatly limits their thermal mass. In this context, insulation becomes the key factor in improving summer comfort.
Thermal mismatch: a key indicator
Thermal lag measures an insulator's ability to slow the transfer of heat.
The performance of insulation in unused attic spaces
In unused attic spaces, there is a significant difference between insulation materials:
- cellulose insulation thermal lag of approximately 9 hours
- mineral wool: thermal lag of approximately 5 to 6 hours
A high phase shift allows the peak heat load to be shifted to the evening or night, which improves indoor thermal stability and makes it easier to cool the building.
Conversely, a shorter phase shift causes temperatures to rise rapidly during the day.
Nighttime ventilation and thermal flow management
A high phase shift allows you to take advantage of nighttime ventilation. Heat reaches the wall later, when outdoor temperatures have already dropped.
Under these conditions, heat flow can reverse, which helps to naturally cool the building.
Feels-like temperature: an operational indicator
Summer comfort depends on the perceived temperature, which is the average of the air temperature and the surface temperature of the walls.
For example:
- air temperature: 19°C
- wall temperature: 21°C
- feels like: 20°C
Hot surfaces, especially ceilings, directly reduce comfort, even if the air temperature remains moderate.
The Crucial Role of Insulation in Unused Attics
In most cases, the roof provides very little thermal mass. Heat quickly penetrates the attic and radiates toward the ceilings.
In this context, insulation becomes the key factor in limiting heat transfer into living spaces.
It is therefore essential to choose an insulation material with high thermal inertia in order to slow down the transfer of heat into the building.
Technical Comparison: cellulose insulation Mineral Wool
Behavior during the summer months
For equivalent thermal performance, insulation materials behave differently in the summer:
- cellulose insulation high thermal resistance, better thermal stability, compatible with nighttime ventilation
- mineral wool: lower thermal resistance, faster temperature rise
The choice of insulation directly affects thermal comfort during the summer.
Key points for your construction projects
- give preference to insulation materials with high thermal inertia
- think in terms of phase shift, not just thermal resistance
- Incorporate nighttime ventilation into the overall strategy
- monitor the temperature of the walls, especially the ceilings
Conclusion: Toward a Dynamic Approach to Insulation
Summer comfort has become an essential consideration in the design and renovation of buildings.
Thermal resistance alone is not enough to ensure comfort. The dynamic behavior of materials—particularly their thermal inertia and phase shift—must be taken into account when selecting insulation solutions.
