Fix or Replace?

Louisville has a lot of leaky windows! It's probably why many property owners assume their energy-efficiency efforts should start with replacing their windows. New windows are expensive. Make that decision in the context of the other energy-efficiency measures you may need to do, since they may save more energy and money.

Fix Existing Windows

If you're handy, you can make your windows tighter and more functional, using these steps:

Remove operable sashes, clean the opening, replace broken ropes and glazing compound (if needed), install new weatherstripping and reinstall. Get more details from, This Old House.
Add storm windows, preferably with a low-e coating. Choose a model with a screen, too, if you want to let in fresh air. (Don't forget to close storm windows in winter.)

Replace with New Windows

While expensive, Energy-Star rated replacement windows offer benefits beyond curb appeal:

With Argon gas between the panes and "low-e" coatings, they have an R-factor of more than 2X that of the tightest fitting older double-pane windows or single-pane windows with storm windows. Their R-factor is more than 4X better than older, single-pane windows without storms.
The clear Argon gas between the glass blocks over 90% of UV light that bleaches rugs and upholstery over time.
They're eligible for tax credits.
New windows with tilt-in sashes are much easier to clean.

Choose windows with wood or Fiberglas frames. (Both can be made with exterior aluminum cladding in a range of colors.) Avoid vinyl-framed windows; they cost less, but are much more prone to warping.

Don't buy triple-pane windows for our climate. The added expense can't be justified here.

If you can't afford to replace all of your windows, consider replacing the ones in the greatest disrepair, and perhaps the others on that same face of the house or building.