Consider exterior or interior storm windows as an alternative
to insulated glass. A well-built storm window system with single
paned windows has been shown to be as energy efficient as replacement
Storm windows are an awfully expensive substitute for caulk.
Caulking, weather-stripping, and reglazing are all inexpensive
do-it-yourself procedures that should be done WHETHER or NOT
you buy storms. After you’ve stopped the air leaks, you
may find that storm windows are not a high priority.
Also be aware that adding storm windows merely changes the
R-value from .9 to 2.0. (The average uninsulated wood-frame
wall is R-4.5)
Poor fitting, single-pane windows can lose energy much faster
than modern multi-glazed low-e windows. In extreme weather,
such a poor performing window may lose energy ten times faster
than an equal area of wall. On clear summer days, an unshaded
window adds greatly to undesirable solar heat gain. In many
homes, heat loss and gain through and around windows accounts
for more than 25% of a home's heating and cooling bills. .
. . . Even though storm windows add little to the insulating
performance of single-glazed windows (that are in good condition,)
field studies have found that they can help to reduce air movement
into and out of existing windows, and thus reduce heating and
cooling energy bills.
Efficiency and Renewable Energy, U.S. Department of Energy