Windows, being part of the building envelope, can contribute up to 30% of heat loss and gain because they can pose a triple threat -- heat moves through a window assembly by conduction through the glass, convection/leakage through gaps and cracks in the framing and assembly and joint with the wall, and by radiation by pulling heat from warm room-temperature objects including people – that’s why when you stand next a window you feel cold in the winter and warmer in the summer (heat moves to the colder space, so in the summer that would be you and the room you are occupying, and in the winter that would be away from you and the room you are in to the outdoors). But remember, replacing the windows themselves is not enough if you don’t also ensure adequate insulation around them by following the insulation and sealing guidelines we set out above. See our Article Window, Skylight and Door Basics -- Increasing Energy Efficiency for a in-depth look at window efficiency.