In the course towards a safe future hydrogen based society, one of the tasks to be considered is theinvestigation of the conditions under which the use or storage of hydrogen systems inside buildingsbecomes too dangerous to be accepted. One of the relevant scenarios, which is expected to have arelatively high risk, is a slow (and long lasting) hydrogen release from a vehicle stored in a closedprivate garage without any forced ventilation, i.e. only with natural ventilation. This scenario has beenearlier investigated experimentally (by M. Swain), using He (helium) to simulate the hydrogenbehavior. In the present work the CFD code ADREA-HF is used to simulate three of theabovementioned experiments, using the standard k- turbulence model. For each case modeled thepredicted concentration (by vol.) time series are compared against the experimental at the given sensorlocations. In addition the structure of the flow is investigated by presenting the helium concentrationfield.