The thermal and moisture conditions around slab-on-ground structures differ considerably from those around the rest of the building envelope. The performed study included laboratory tests and in situ surveys of slab-on-ground structures as well as theoretical calculations and numerical analyses concerning the moisture behavior of typical ground floor structures in Finnish climate. The results show that the soil mass under slab-on ground structures is usually warm and moist enough to cause a water vapor diffusion flow from the moist base soil towards the relatively dry indoor air. The moisture level of the base soil and the drainage layers under the slab remain high thanks to capillary forces, diffusion and excess penetration water. The heated building mass warms up the subsoil in a relatively short period of time despite the thermal insulation of the floor structure and the low average outdoor temperatures during the Finnish winter season. The diffusion flow through the ground floor must be taken into consideration during the planning and construction process of slab-on-ground structures, and especially when choosing the coating materials. In every construction phase the slab-on-ground structure must be able to dry out to some direction and coating materials must be sufficiently permeable to the potential water vapor flow to prevent relative humidity from rising above the harmful critical level in any structural layer inside the floor structure.
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