In this study, heat transfer through a double-pane window is numerically analyzed by a finite difference technique. The aim of the study is to determine the thermally optimum air-layer thickness between the two panes for different climates. Four different cities of Turkey, representing different climate conditions are considered: Ankara, Antalya, Kars and Trabzon. The height of the window, H is chosen 80 cm. The effect of air-layer thickness varies between 3 and 40 mm on the average Nusselt number and the heat flux through the inner pane. It was shown that energy losses through the double-pane windows can be considerably reduced by optimizing the thickness of air layer.