Energy Saving Windows
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
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.