In this study, the optimum air layer thickness of double-glazed windows is determined using the degree-days method. Calculations are obtained for Iskenderun, Kocaeli, Ankara and Ardahan which are in different climate zones of Turkey. Heating cost of the objective function is calculated for natural gas, coal, fuel-oil, electricity and LPG. The optimum air layer thickness is obtained for three different base temperatures which are 18,20 and 22 degrees C. The results show that the optimum air layer thickness varies between about 12 and 15 mm depending on the climate zone, fuel type and base temperature. The effect of the fuel type and the base temperature on the optimum air layer thickness diminishes in cold zones. It is shown that with a well-optimized glazed window, up to 60% energy saving can be achieved. (C) 2010 Elsevier B.V. All rights reserved.