The influence of ground slab permeability on wall moisture in a historic building

When impermeable ground bearing slabs are installed in old buildings without a damp-proof course, it is a common belief of practitioners within the conservation industry that ground moisture will be ‘driven’ up adjacent walls by capillary action. However, there is limited evidence to test this hypothesis. The accumulation of moisture in walls can promote the decay of the wall materials, decrease the thermal performance of the building envelope and adversely affect the comfort and health of occupants.

An experiment was used to determine if the installation of a vapour-proof barrier above a stone flag floor in a historic building would increase moisture content levels in an adjacent stone rubble wall. This was achieved by undertaking measurements of wall, soil and atmospheric moisture content over a three-year period. Measurements taken using timber dowels showed that the moisture content within the wall did not vary in response to wall evaporation rates and did not increase following the installation of a vapour-proof barrier above the floor. This indicates that the moisture levels in the rubble wall were not driven by capillary rise.