Heat loss through windows in poor condition can reach 10-20%, but contrary to what most everyone thinks investing in new energy efficient windows is one of the least productive ways to spend money to save energy. The payback period (the time it would take to recoup your investment) for energy efficient windows is between 20 and 30 years. (You would be better off sealing leaks in your foundation, basement and attic, or adding insulation, especially to your attic.

Meanwhile there are many things you can do with the windows you have that can save substantial amounts of energy—and money.

What you can do:

• Make sure that windows are latched securely
• Install storm windows if you have them
• Rope caulking and weatherstripping
• Repair any broken or loose panes of glass in windows
• Install plastic sheeting on the interior of windows
• Close drapery or blinds to stop cold from coming in
• Open blinds and drapery on sunny days to take advantage of the sun
• Purchase and install storm windows (especially on west and south sides of home)
  

To help you decide which windows require your attention you can try to detect the air leaks in each window. Do your windows rattle in a heavy wind? Do your curtains move around even in a gentle breeze? If so they require rope caulking and weatherstripping.
And when you work on your windows consider doing them in this order: The west-facing windows are most likely to catch the prevailing winds, so do them first; the north-facing windows will be exposed to the coldest air, so do them second. The east-facing windows are next. Southern windows are probably the least vulnerable, because they face the sun, so they can be done last.

If you aren’t going to be replacing your windows any time soon you might consider purchasing and installing storm windows. Storm windows can cut down heat loss from each window by more than 60%, by reducing the impact of the wind and by increasing the R value of each window.

Savings:

If you can address all the steps on the list above (with the exception of the last one) you can expect to save 5-10% of your heating costs, depending on the condition of the windows and the rest of the house when you started.