Moisture in a drying concrete slab redistributes subsequent to installing semi-permeable flooring, hence increasing the surface humidity beneath it. Such a humidity increase may be substantial in terms of relative humidity, RH, even though the redistributed moisture content appears insignificant. This comparably large increase originates from the hysteresis, moisture history dependence, exhibited by sorption isotherms for cement based materials. In contrast to retracing the desorption isotherm pursued when drying, the material moisture content traces a scanning curve, when adjacent humidity increases. Scanning curves have a lower moisture content change in relation to RH change, moisture capacity, compared with the desorption isotherm. A qualitative and quantitative model of moisture redistribution in a dual layer concrete slab with regards to RH distribution and scanning curves are presented. The model is validated by determining the RH distribution in a number of screeded concrete slabs, before flooring and after a certain time of redistribution. Sorption isotherms and scanning curves for four cement based materials are shown.