Windows and doors, being a large 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, and in the summer that would be you and the room you are occupying).  Addressing energy loss issues with windows and doors can have a dramatic impact on your energy consumption and heating and cooling bills.

Whether or not you need to replace your windows depends on the type and condition of your existing windows.  If you already have insulated glass windows and the sashes are working then you may not need to replace them, even fogged windows can often be repaired, but the windows should have a thermal break and frames of aluminum or wood usually have the best energy performance.  This Article will tell you how to improve the efficiency of existing windows and how to choose new windows if you need to replace what you've got, starting with the basics of window design and energy saving features.