One of the easiest and least expensive ways to improve window efficiency is the build an interior storm window. The frame is made out of 1x2s and covered with flexible plastic on both sides to create a trapped air space in the middle. The total cost for this window was $5 or $6. It is amazing the difference that can be felt when just standing next to the uncovered and then covered window in cold weather. When uncovered you can feel the cold coming off of the window. I think that this simple frame about doubles the R value of the existing single pane window. This is the next window to be replaced with a vinyl frame double pane unit.