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