If a leakage of CO2 out of a geological reservoir were to happen and to reach the vadose zone below a building, CO2 could migrate through the vadose and the building's slab and accumulate in the building, leading to possible acute risk for the inhabitants. A representative-scale experiment, including a prototype for a building, was developed to better understand and quantify this possible risk. It brought fruitful directions for further modeling work, since unexplained CO2 peaks were observed in the prototype. Numerical simulations were carried out to address the variability of CO2 concentrations considering the influence of soil and building properties as well as meteorological conditions, with promising results for risk analysis.