The main characteristic of an insulating material is its thermal conductivity. The capacity of a material to be left to a minor or major extent by the heat flux is determined by the coefficient of “thermal conductivity”, called “lambda” (λ) and is expressed in W / mK. both the material is more insulating. Lambda is a standardized measure and is conventionally declared and measured at 10 ° C.
Gases are the elements that have the best thermal insulation coefficient. On this basis, fibrous materials such as glass mineral wool that are embedded between air fibers, can reach insulation values very close to the immobilized air.
The greater the thickness of an insulating material, the greater its ability to withstand the flow of heat.
This characteristic is called thermal resistance (R) and represents the ratio between the thickness of the insulating material and the lambda value (λ) of it. Expressed in m2K / W.
The higher the R value, the more the product is opposed to the heat transfer resulting in better insulation.
For example, the thermal resistance of an insulated floor with mineral wool plates with λ = 0.034 W / mK with a thickness of 15 cm is 0.15 / 0.034 = 4.41 m2K / W.
In order to achieve the same thermal resistance using an isolated ceramic block we must make a wall with a thickness of 62 cm.If we were to use wood, for example, in order to obtain the same thermal resistance, we would have to use an 80 cm thick layer.
From the table and the examples of calculations it is clear that in order to have an effective thermal insulation wool must be used as insulation material. There are other insulation materials but most are made of plastics and are combustible plus they do not provide a good resistance over time to the mineral materials.