For the glass area, triple gives you a better R-value than double, about
5 versus 3.5.
HOWEVER, the frame needed to support three panes of glass is a better
thermal bridge than what is needed for double. On many windows, the gains
in R-value for triple is offset, or WORSENED by the frame.
If you must have a large picture window, a triple-glazed window would likely
make a big difference. Of course, shutters would also make a big, or even
bigger difference as well.
In our home, our largest windows are 54" x 54" casement (two-section) and
it just made no sense to go with triple-glazing.
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