Moisture behaviour of slab-on-ground structures in operating conditions: Steady-state analysis

This paper examines the moisture behaviour of slab-on-ground structures in moist steady-state conditions where the capillary phenomenon is excluded and water vapour diffusion is the only process involved. The focus is particularly on the impact of the temperature of the subsoil, the relative humidity of the indoor air and the water vapour resistances of the floor covering and thermal insulation. The subsoil or fill surface beneath a heated building is warm and moist (relative humidity, RH, near 100%). As a result, the water vapour diffusion frequently flows from the subsoil towards the drier indoor air, especially when the relative humidity of the indoor air is low. The critical section in slab-on-ground structures is usually the relative humidity on the lower surface of the floor covering. The moisture behaviour of the structure depends on the water vapour gradient across the structure and the water vapour resistances of the structural materials, especially of the floor covering. Adequate thermal insulation beneath the concrete slab stabilises the temperature gradient between the subsoil and the slab-on-ground structure and decreases both the water vapour gradient and inwardly-directed diffusion flow. A smaller water vapour gradient is a precondition for safe use of water vapour-tight floor covering materials. The higher water vapour resistance of thermal insulation also decreases the relative humidity at the slab/floor-covering interface.