Energy Saving Windows

The most frequent reason for the addition of interior or exterior storm windows is Energy Conservation and the resulting reduction of heating and cooling costs. Industry guidelines indicate that the addition of a storm window to an existing single-glazed window will reduce the energy loss through the window area by approximately 50%. This savings applies to both heating and cooling.

The energy efficiency of windows is normally measured using a U-Value, which is the “tendency” of heat energy to flow through the total window system. For example, the U-Value of a standard single-glazed wood double hung window is about 1.12. The addition of a storm window will reduce that U-Value to about .50-.58, depending on the type of storm window which is used. Thus, the 50% reduction in energy losses.

There are two (2) components to the total U-Value of a window system:

» Uc is the tendency of the window system to lose energy by “conduction.”

» Ui is the tendency of the window system to lose energy by “infiltration.”

Total U-Value = Uc + Ui

Storm windows are highly effective because they provide an insulating air gap to reduce “conduction” and an additional airstop to reduce “infiltration.”

Various glazing materials can reduce the U-Value even further, but at least 80% of the energy savings comes from having the basic storm window there to provide “secondary glazing.” The most popular energy-saving option is low-e glass, which reflects heat back into the building in the winter and back outside in the summer. Window films can result in significant energy savings, providing sun relief in the summer, and low-e benefits in the winter.