It depends. Windows themselves have such a low R-value (resistance value), that investment to return in a weatherization aspect alone isn't great. However, going from a single-pane aluminum framed window to a newer double-pane vinyl window will have much better effects on your home's cash value. In any weatherization job, air sealing is much more effective than changing out a window. With that being said, there are specific cases where changing out windows is imperative to reducing energy costs, mostly in mobile homes.