Effect of interior and exterior insulation on the hygrothermal behaviour of exposed walls

Massive walls exposed to the natural climate without special rain protection show a dynamic moisture equilibrium governed by the alternating events of rain and sunshine. The moisture further reduces the rather low insulation level of the walls. Therefore, thermal insulation measures have to be taken in order to improve the hygienic conditions and the thermal performance of the building. The influence of interior and exterior insulations on the hygrothermal behaviour of such walls can be determined with the aid of a recently developed computer program which has been experimentally validated by comparison with field tests. The results show that an exterior insulation leads to the drying of the wall, with the drying rate depending on the vapour permeability of the insulation system. An interior insulation, however, results in a rising water content of the wall due to the decreasing masonry temperature. This effect is almost independent of the vapour permeability of the insulating material. While an exterior insulation also improves the thermal resistance of the masonry, an interior insulation has the opposite effect and increases the frost damage risk. Therefore, the interior insulation of exposed walls should be combined with rain protection measures at the facade.