The embodied energy of the four main materials used in the construction of an inert gas filled, double-glazed window, of a given size, was quantified. The four main materials considered were infill gas (argon, krypton and xenon), timber, aluminium and glass. The energy requirements of the underlying manufacturing processes are also estimated for the processing of the finished product. The total embodied energy for argon, krypton and xenon cavity filled windows (1.2 m by 1.2 m standard tilt and turn window), was found to be 1031 MJ, 1539 MJ and 5531 MJ per window, respectively. For argon filled cavities, this leads to the production of 94.7 kg of CO2, 1.2 kg of SO2 and 0.4 kg of NOx per window. The results presented in this paper are initial findings in the first stages of a complete Life Cycle Analysis (LCA) of double-glazed windows for sustainable buildings.