I will have to dig out my HVAC manuals to check but seems that the
table showing R values for dbl pane windows indicated that the bigger
the air space the better the R. seems like the triple pane were very
impressive. My previous house (central TX) had storm windows
installed. They created not much more than a primitive dble pane, maybe
a couple inches of air gap. Never had any condensation problems but I
am sure they are not very airtight. Wasn't insulation value the
marketing hype back when these things were sold? Guess they are
obsolete now with dbl pane windows so prevalent.