In shallow tunnelling below groundwater table compressed air can be used for preventing water inflow into the tunnel during construction. The required excess air pressure in the tunnel is being produced by a compressor station installed on site. The production of the compressed air can be very cost intensive. During tunnelling with a shotcrete support airloss takes place which has to be compensated permanently to maintain the required air pressure. The air leaks thereby through the unsupported tunnel face and the shrinkage cracks of the shotcrete support. Within the present work experimental investigations for the determination of the air permeability of the shotcrete cracks and the adjacent ground have been carried out. On the base of the experiments and theoretical considerations there has been determined a calculation method to determine the airloss through the shotcrete lining. The new developed calculation method allows the design engineer to determine a more realistic amount of airloss for tunnels with a shotcrete support driven under compressed air. So the efficiency of this method can be increased.