In order to design a moisture safe slab on ground, a minimum temperature difference of approximately 3 degrees Celsius between the floor and the soil is often recommended. For a slab on ground with floor heating, the heat loss to the ground is greater than in a regular slab. However, the extra heat loss during the heating season results in a warmer soil. This may lead to an upward directed moisture flow during periods when the floor heating is switched off, which can cause moisture damages. In this paper, a new theoretical model that predicts the dynamical hygrothermal conditions in the foundation is presented for 3-d cases. The model can be used in the design of moisture safe foundations.