These questions and concerns are the top reasons I have found that people are looking to replace their existing windows. I will add one more. If your home is new and you bought it through a builder, chances are your brand new windows are not as energy efficient as they should be. Most new construction builders provide what we call a "builders grade" window, mainly because of the costs associated with construction. These windows are not designed to perfom to the levels necessary to keep your home energy efficient. Some of the things to ask your builder are: Will our windows be energy star rated? Will our windows include Low-E Glass and Argon Gas? Can we pay a little more for substantially better windows?
Windows and doors account for about 50% of the energy loss in your home. 40% is lost through the ceiling into the attic and about 10% throught the floors. Proper insulation is key in helping your home remain efficient and comfortable. Think of your exterior walls as a barrier of protection. If your windows are weak you are losing valuable energy to the ouotside. In turn you will find yourself uncomfortable and your air conditioner and furnace will be working double time to keep ub with the changes in inside temperature. Your windows are also supposed to keep out the outside temeperatures whether they be too hot or too cold. Proper glass in your windows can add protection against the outside heat by about 90% and the same with the cold. Vinyl is one of the best materials invented to combat cold transfer because it does not conduct like aluminum does and it will insulate even better than wood because if can be designed with numourous air pockets through out the window frame. If you have aluminum windows in your home, I will always recommend changing them out to vinyl. You will see a substantial differnce in your energy costs and the over all comfort of your home.
if you have windows that are a single pane of glass you are only protected against birds flying in. Single pane windows do absolutely nothing to insualate your home from the outside elements, sure you won't get rain in the house but most of the time a single pane of glass will actually magnify the heat coming through and also the cold.
In conclusion, if your home feels too cold or too warm, if your gas bills fluxtuate substantially from one season to the next, if you look around and you are living in the only house with ugly windows it might be time to upgrade your windows.