Storm windows
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Storm windows are windows which are mounted outside of the main glass windows of a house.
Most commonly, they are found in cold climates to serve as a retrofit on existing windows in order to improve their thermal efficiencies. Similarly, storm doors (also called "screen doors") allow similar winter savings with older inefficient doors - and allow a screen for summer ventilation.
Storm windows can range from rigidly mounted glass exterior windows to mounted plastic sheets which merely serve to cut the wind exposure of the inner glass windows - and therefore the wind chill experienced by the glass.