Certain properties of building materials play a decisive role in summer comfort. It is therefore advisable to take them into account in building design and renovation projects, especially in warmer regions.
Among these properties, thermal inertia represents the ability of a wall or floor to store heat and release it gradually. It plays a key role in summer comfort because it helps mitigate temperature variations indoors and maintain a stable temperature throughout the day.
The thermal inertia of a material depends on its intrinsic properties: its diffusivity and effusivity. These two properties are themselves linked to three factors: the density, specific heat capacity, and thermal conductivity of the material.
1. Thermal conductivity
Thermal conductivity is the energy that passes through onesquare meter of a material one meter thick when there is a 1°C difference between the two sides.
It characterizes a material's ability to transmit heat.
It is indicated by the Greek letter lambda and is expressed in W/m.K.
The lower the lambda value, the less heat the material will transmit.
2. Density
Density, also known as mass per unit volume, is the mass of one cubic meter of insulation.
It is expressed inkg/m³.
The higher the density, the longer it will take for heat to pass through the material.
3. Specific heat capacity
The specific heat capacity is the energy required to raise one kilogram of this material by one degree Celsius.
It characterizes the material's ability to store heat.
It is expressed in J/kg.K.
The higher the mass thermal capacity, the more heat the material can store.
These three indicators are used to calculate the effusivity and diffusivity of a material.
Effusiveness
Thermal diffusivity expresses a material's ability to exchange energy with its environment. It characterizes a material's ability to absorb and store energy.
It is expressed in J.K-1.m-2.s-1/2
The greater the effusivity, the more positive the impact on summer comfort.
Diffusivity
Diffusivity, on the other hand, expresses the speed at which heat diffuses through a material. It characterizes a material's ability to transmit heat quickly.
It is expressed inm2/s.
The lower the diffusivity, the more positive the impact on summer comfort.
It is the combination of these characteristics and the building design that will have an impact on summer comfort.
When it comes to summer comfort, it is essential to consider several factors when choosing insulation materials. The criterion cannot be limited to thermal conductivity, known as lambda.
For wall insulation, French construction principles favor dense walls using materials such as cinder blocks, concrete, and brick. The interior temperature of the walls is generally close to the ambient temperature.
On the other hand, French roofs, where the roofing and framework are the only elements of the outer envelope, mainly use materials with low thermal inertia (tiles, slates). Heat penetrates directly into the attic, where the interior temperature can reach twice that of the outside. This heat radiates directly onto the floor of the attic (the ceiling of the house) and thus warms this wall (effectively increasing the perceived temperature).
In this configuration, only insulation can provide this thermal inertia and act as a shield against the heat accumulated in the attic. The choice of insulation is therefore essential, and must be sufficiently effective to guarantee comfort in both winter and summer.
