- Thermal capacity of building materials
- Thermal inertia to regulate the temperature
- Thermal phase shift to protect yourself from the heat
Generally, when we talk about insulation, we think protection against the cold. But a good insulation must also avoid solar heating inside the house to limit the use of air conditioning. It must also make it possible to regulate and homogenize the internal temperature. Depending on where you live, insulators do not have the same role to play.
The qualities of a good insulation
Thermal capacity of building materials
Each material has a way of behaving in front of its own heat.
Thermal capacity is the amount of heat that a material can store in relation to its volume. " It is defined by the amount of heat required to raise the temperature of 1 cubic meter of material by 1° C ».
The denser a material is, the more it can store heat and the higher its thermal capacity. A light material will, on the contrary, have a low thermal capacity.
For example, to isolate a roof is better to use a dense material which will protect the interior of the house not only from the cold but also from the summer heat.
Thermal inertia to regulate the temperature
Thermal inertia depends on the thermal capacity of the material. A concrete, brick or stone wall that is insulated has thermal inertia.
Thanks to the thermal inertia, the wall will restore the stored heat or freshness and so regulate temperature differences between day and night. Thermal inertia is an important element of comfort.
The thermal inertia of a material depends on its effusiveness and diffusivity:
- The effusivity of the materials. Effusiveness is the ability of a material to absorb or restore more or less rapidly the heat it has stored. A material with high effusivity value stores a lot of heat, but appears cold on the surface. This is the case of stone and marble in particular that are cold to the touch, but have a good thermal inertia. On the contrary, a material with low effusivity value does not store heat, but it seems hot on the surface. This is the case of wood or cork. They are pleasant to the touch but they do not have thermal inertia.
- The diffusivity of the materials. Thermal diffusivity is the rate at which heat is propagated in a material. It depends on its conductivity ((λ - lambda) and its thermal capacity.
Thermal phase shift to protect yourself from the heat
An insulator must be able to avoid heat loss during the cold season but also avoid the rapid heating of the house in the summer. Good insulation depends on the inertia of the materials used and their thermal phase shift capability.
Thermal phase shift is " the time it takes for the heat to penetrate inside the habitat ».
This is a key element for summer comfort. The longer the duration of the thermal phase shift, the more the house will stay cool during the day.
The phase shift coefficient materials depend on their thermal capacity. The denser the material and the higher its thermal capacity, the higher its thermal phase shift coefficient.
In regions where the temperature regularly exceeds 30°, the ideal is that it is at least 10 hours. Thus, heat does not arrive in the house until the end of the day. Good ventilation, at night when the temperature is the lowest, then naturally refresh the rooms.
Thermal inertia and thermal phase shift are two key concepts in bioclimatic habitat design. They allow the indoor temperature to self-regulate naturally according to the seasons.
To read also: How to renovate the insulation of a house?