The formation of a flammable hydrogen-air mixture is a major safety concern especially for closed space. This hazardous situation can arise when considering permeation from a car equipped with a composite compressed hydrogen tank with a non-metallic liner in a closed garage. In the following paper a scenario is developed and analysed with a simplified approach and a numerical simulation, in order to estimate the evolution of hydrogen concentration. The system is composed of typical size garage and hydrogen car's tank. Some parameters increasing permeation rate (i.e. tank's material, thickness and pressure) have been chosen to have a conservative approach. A close look on the top of tank surface showed that the concentration grows as square root of time, and does not exceed 8.2 10-3 %2by volume. Also, a simplified comparative analysis estimated that the buoyancy of hydrogen-air mixture prevails on the diffusion 35 seconds after permeation starts, in good agreement with simulation where time is at about 80 seconds. Finally, the numerical simulations demonstrated that across the garage height, the hydrogen is nearly distributed linearly and the difference in hydrogen concentration at the ceiling and floor is negligible (i.e. 3 10-3 %2.