What to Look for
in Energy-Efficient Windows
by Alex Wilson
Windows account for a huge percentage
of heat loss in the typical New England
home. Even though most houses now incorporate
insulated (two-layer) glass, this
still provides an R-value of only about 2
for window areas—hardly a match for the
R-20 to R-26 going into walls.
Further, the R-value tells only part of
the story: the amount of heat lost by conduction.
Windows also are major sources
of infiltration (convective) heat loss.
Most estimates suggest that windows
account for 20 to 25 percent of the heat
when other considerations rule out
Historic-restoration projects, for example,
may call for traditional 12-over-12
windows. Using applied grills over
insulated glass is unacceptible