Are Crittall's steel-framed windows of yore that much worse for
insulation than cheapo plastic-framed double-glazed units?

In terms of heating bills, how long would it take to amortize the cost
(ca. £5,000) of replacing steel-framed windows with the uPVC variety?
I reckon it would take literally years before one had paid off the
costs through fuel savings.

What if the steel-framed windows are furnished with secondary
double-glazing? Doesn't this make them equal to uPVC?

MM