The Italian law sets performance requirements for the summer energy need due to the building envelope. Since the calculation is based on a quasi steady state method, it involves the use of a dynamic parameter, i.e. the loss utilization factor for cooling (C). This parameter, that is a function of the ratio heat losses over heat gains (1/GLR) and of the building thermal response (Van Dijk 1987), should represent the ratio of the utilized vs. total heat losses in a building. In this paper, the influence of weather data on the value of the loss utilization factor and the reliability of a quasi steady state approach for the prediction of cooling loads are analyzed.