For some years I lived in a rowhouse in the Maryland suburbs of DC, built about 1983. The units there had double glazing. Maryland does not have an especially c

For some years I lived in a rowhouse in the Maryland suburbs of DC, built about 1983. The units there had double glazing. Maryland does not have an especially cold climate. Where I live now we have single panes, but behind storm windows. On the other hand, we know a family near Boston who live in a big old house. The husband, a mathematician, calculated that they would never recover the cost of replacing the windows. I don't know how far out he calculated, but I expect it was about 20 years.