If your home is leaky and woth low levels of insulation, installing new energy-efficient windows may not significantly reduce your energy consumption.
In other words: replacing old windows in homes with low or moderate levels of insulation may not be cost-competitive and advantageous, unless of course the windows are leaking water into the walls or aren't working properly; or unless you first carry out significant improvements in your home's insulation and sealing.
Bottom line: investing in energy-efficient windows can be a very good investment, that you should consider; but only...
1) In new homes,
2) In apartments and
3) In homes with high levels of insulation and sealing.
Energy efficient windows in new homes
According to data from the National Association of Home Builders, the average cost of new windows is about $6.000 (USA, Canada), that is, about 3,3% of the overall cost of the house.
On the other hand, heating and cooling costs in American homes (according to the EIA) amount to an average of $1.300 per year, while unwanted energy losses and heat gains through windows are often estimated at about 1/5-1/3 of the energy bills (EERE).
If we consider these numbers, it’s easy to conclude that in new homes, an additional investment of $2.000 to $3.000 in new energy-efficient windows is something that can be easily recovered.
The paybacks can be relatively short, and the table above shows it. For example: an additional cost of $2.000 can be recovered in 7 years, for energy savings of about 20%.
Window replacement in apartments
The issue is a bit different in apartments.
Their walls, ceilings and floors are often "protected" by the walls and floors of the contiguous apartments; that gives them a thermal protection that amounts to reasonable or good levels of sealing and insulation - a reason why many apartments do not have the serious thermal problems of single-family homes.
And since old and energy-inefficient windows become the most important source of unwanted heat gain and loss, and they do not typically involve a large area, replacing them can be a very cost-effective investment.
That’s not a rule, of course. There are other means of reducing energy bills in apartments (window films, shutters, blinds in the summer; storm windows and home insulation kits in the winter…); and some of them are less expensive and can also provide significant energy savings. But there are many cases where the replacement of old and inefficient is in fact advantageous, from an economic standpoint, in both cold and hot climates.
Replacing windows in existing homes
If your home has good sealing and insulation levels on its walls, ceilings and floors, your old and energy-inefficient windows are very probably the biggest source of energy loss and unwanted heat gains. And it’s not difficult to figure out that their replacement is a rewarding investment, if your heating and cooling costs are high.
But, as already mentioned, that's not the case of replacing old windows in homes with low levels of airtightness and insulation. You have to improve your home's insulation and sealing first. Otherwise, even in the best-case scenario, the payback will be very long.
New windows, in such scenario, may block some drafts and air leakage, and reduce unwanted heat loss (or heat gains), but leaks and lack of insulation in other parts of the home’s envelope will reduce most of the energy gains provided by windows.
People often feel disappointed with their new windows, when they do not solve condensation and other thermal problems; but that is explained by the fact we have just mentioned – or by cheap replacement windows that do not add much to the old units.