A thermal insulation or insulation is a material having a low thermal conductivity, that is to say a material capable of opposing the heat flux passing therethrough, a high thermal resistance of conduction. In construction, insulation provides good thermal insulation by reducing heat leakage (cooling) or heat entry ("keep cool"). High-performance thermal insulation results in the use of low thermal conductivity (λ) insulation (up to 40 cm insulation with a thermal conductivity λ = 0.04 W / (m ° C) according to criterion Of the passive houses makes it possible to obtain a thermal resistance surface area of 10 m².K / W). In addition to the overall performance of the walls, it is necessary to avoid the presence of thermal bridges. Thermal insulation is one of the most cost-effective energy-saving investments, especially in new construction and can be considered as renewable energy equipment with the longest service life (more than 100 years for good Insulators of opaque walls).
In the northern countries, passive buildings (Passivhaus or Minergie criteria) are built in energy, notably in Germany, the Netherlands, Austria and Switzerland, which no longer require a boiler. Better still, positive energy constructions produce more energy than they consume (especially by exporting electricity to public electricity grids). Thermal conductivity is a determining factor in the winter heat exchanges of the walls, but in thermal summer exchanges of the walls undergoing solar radiation, thermal diffusivity is paramount.