Estimating thermal effect of so called double-skin façades on energy performance of buildings with two envelopes, a proprietary mathematical model of heat transfer through double-skin façades has been developed. A very specific approach of the mathematical model presented here is based on prediction of the double-skin façades interspace temperature and assumption that façade could be treated as a single one declaring the interspace temperature and reduced intensity of solar radiation as the “outdoor” conditions. In this paper results of the heating and cooling loads as well as the interspace temperature prediction compared for three months (January, April and July) and for the west oriented façade with ordinary and absorption glass are presented. Façade is placed under climatic conditions of Belgrade (45° North Latitude). Also, results for weather conditions of sunny and cloudy day are shown. Results includes influence of air velocity in interspace of façades for April and July. Following a type of the double-skin façade construction assumption it was implied that the space between the two envelopes is closed during the winter mode for air circulation, in order to have temperature higher inside than outside and opposite in the summer mode, air inlet and outlet are opened in order to prevent much higher temperatures than the outside ones, and higher cooling load of an air-conditioned building, as a consequence. Taking only thermal effects into account, presented prediction has proven that double-skin façades decrease both building heat losses and heat gains throughout the year, and give significant contribution to the building energy consumption savings.