External walls – together with windows – are generally the largest cooling surface of the building. Therefore the insulation of these walls can significantly reduce the heat loss and increase the comfort. From building physics point of view the optimal solution is to install the insulation on the external face of the building envelope: this way the risk of interstitial condensation is minimised, the existing structural walls also get thermal protection and the heat storage capacity of the massive walls are utilised.

However, in certain cases the external application of an insulation layer is not possible, or the internal use of an insulation layer has more benefits. When upgrading existing buildings it is not always possible to apply the insulation externally, especially if the outside appearance of the façade must be retained. In the case of existing walls with direct contact to the ground, the internal wall insulation is often the only way to improve thermal insulation. Internal insulation has significant advantages in rooms for temporary use, not heated permanently: those can be heated with minimum energy consumption.