- What is the thermal inertia of a material?
- The advantages of a well controlled thermal inertia.
- Some ways to improve the thermal inertia of a building.
Thermal inertia is a key point to consider when designing a building or renovating it. Proper to each material, it is a physical law that, well used, allows to limit a large part of the temperature variations with, in the key, savings in heating and air conditioning. Not to be confused, however, with insulation, although both are often linked. The thermal inertia to its own identity and an insulator can display good thermal results with average inertia.
What is the thermal inertia of a material?
Thermal inertia is the intrinsic capacity of a material to accumulate heat, or conversely freshness, before returning it. The time elapsing between the accumulation and restitution phases being variable according to the nature and the thickness of the material used. So that there may be an offset, called phase shift, of several hours between the two. An interesting feature that can be used to improve the comfort of the home.
More precisely, the thermal inertia of an element is broken down into three distinct criteria. The thermal capacity corresponding to the maximum amount of heat that can be absorbed by the material in relation to its mass or its volume. The thermal effusivity which represents the rate at which the surface temperature of the material fluctuates and the thermal diffusivity which indicates as for the conductivity, of the heat inside the material itself.
Of course, for the inertia of a material to have an impact on the internal temperature of a room or a building, it must have a relatively large exchange surface such as walls or insulation who covers them. Attention however, only the outer insulation is concerned in this case. It should be kept in mind that isolating a wall from the inside has the effect of "breaking" the influence of its thermal inertia.
The advantages of a well controlled thermal inertia.
Benefiting from a good level of thermal inertia for its building materials is a plus for the comfort of the inhabitants and their wallet. But it must be properly used because, poorly employed, it can have repercussions the opposite of the goal. To obtain a suitable thermal inertia, the chosen material must have a fairly high heat capacity, good thermal conductivity and a relatively high phase shift time.
The ideal being a phase shift of 12 hours to play the alternations day / night. Thus, the energy gain becomes optimal since the heat diffusion times are reversed with respect to the external climatic conditions. In the summer, especially, it does not overheat the indoor air during the day. The diffusion of stored heat is released at night when it is a little cooler, good ventilation aids the structure of the walls to cool for the next day. The need for air conditioning is therefore limited or unnecessary.
In winter too, good thermal inertia is an asset and saves energy. In particular, it allows, by absorption, not to lose all the heat energy generated by a house. It stores everything, heat from the heaters of course, but also from the use of electrical appliances, appliances and residents. In addition, whether in summer or in winters, inertia helps smooth out outdoor temperature spikes by decreasing and spreading their incidences.
Some ways to improve the thermal inertia of a building.
A good thermal inertia is appreciable, otherwise the temperature variations between outside and inside are almost simultaneous and act with the same force. So much so that the difference between the two is no longer or very little felt. What make a house quickly unbearable. So that the thermal inertia can act, a certain thickness is necessary. Sessions that are too fine can not be suitable. And again, this thickness will be a function of the nature of the material chosen. For example, solid concrete block walls should be at least 7 cm wide, hollow concrete blocks should be greater than 11 cm and terracotta monomur rows should be at least 30 cm. In this respect, the interior elements such as the partitions can bring a surplus of inertia by increasing the exchange surface.
Insulators also play a vital role and themselves possess thermal inertia. They should not be chosen lightly. Like the glass wool, so common, which only provides a slight phase shift of 6 hours while the wood fiber reaches 15 hours. And, alongside traditional insulators, PCMs or phase change materials demonstrate a certain potential. They are based on heat exchange phenomena during changes in the state of matter, by integrating elements with low melting temperature. Like plasterboard with paraffin microcapsules melting above 20° C. In this way, during the summer, the day the paraffin contained in the microcapsules passes from the solid state to the liquid state, which has the effect of absorbing part of the heat. Then, at night, when the thermometer goes down, the paraffin becomes solid again while restoring the heat.