By far the most common energy efficiency improvement is to fit double glazing window. This does save energy but perhaps not as much as you might think.
double glazing window
The Energy Saving Trust estimates that an average house would save £120 per year by replacing single glazing with double glazing. BUT it would cost perhaps £5000 – a payback period of OVER 40 YEARS.
However, if your windows are in poor condition, this is the sensible thing to do.
Secondary glazing and draught proofing the same house will only produce savings of about £75 per year. But at perhaps only £1500 per house, the payback can be half that of full double glazing.
And don’t forget draught proofing and thick curtains!