New windows can improve energy savings
There are several things you can do to produce energy cost savings. After you've insulated your loft space the most cost effective is replacement windows.
There are two entirely different ways in which replacement windows can produce energy cost savings. The first is that many older window frames do not fit that well
This means that there are leaks of warm air out and cold air in around them. Fitting replacement windows that fit well can stop this wastage and produce energy cost savings.
Double and triple glazing.
The second way older windows can waste your energy and money is that they are single glazed windows.
The single pane of glass, although glass isn't thought of as a good conductor of heat, allows the cold air on the outside of the window to cool the heated air on the other side.
So replacing such single pane windows with double (or in very cold climates, triple) glazed ones can produce good energy cost savings. The same is also true, in reverse, with hot and very hot climates and air conditioning.
The case for replacement windows and their energy cost savings depends upon temperature differences, not just heat or cold.
When you fit replacement windows there's a thing you can do to maximize energy cost savings. Instead of the 1⁄4 to 3/8 th inch gap common in older replacements look at 1⁄2 to 3⁄4 inch gaps between the panes.
This will increase the energy cost savings the replacement windows provide and it's also much better for noise reduction: a double benefit.
Source: National Energy Foundation.