The article examines the moisture risk associated with the conversion into flats of cellars in historic (100+ years) multifamily houses in a temperate climate for the case of downtown Vienna/Austria. The goal is to use the thermal losses of the flat above the cellar through the ceiling for heating the new living space (-> project title: “Space4Free”, Wegerer et al. 2022) using the tempering properties of the soil. Further framing conditions are acceptable hygienic conditions and thermal comfort. The cellar’s brick walls are not sealed off the soil for technical and economic reasons during refurbishment. The concept includes the installation of a mechanical ventilation plant. A three dimensional simulation of moisture and heat transport was carried out the coupling building parts to the indoor air. Focus was on designing a moisture based control algorithm for the mechanical (automated) ventilation plant which resulted in a decision tree with 8 states of ventilation operation. The simulation showed that such a cellar refurbishment – from a building physics point of view – is feasible. A main result is that the annual moisture immissions are driven by the moisture loads induced by the inhabitants such as cooking rather than by moisture originating from building components which are in contact with the soil. This result is corroborated by the fact that such conversion has been successfully practiced by a company represented among the authors since several years. A practical implication that – in the current case – insulating building components against moisture from the soil is less important than controlling user driven moisture loads.