Some of the most prominent energy thieves in your home are your windows. You might be able to notice a difference in temperature standing close to a window, and the worse shape it is in, the more pronounced the problem will be.
Having single pane windows is one of the worst issues, but most modern homes now have double pane windows that insulate better. However, a window is only as good as the frame it sits in. Many window frames will have cracks and holes in them that let the outside air through. Sometimes you might be able to notice this by shining a flashlight through the frame while someone outside is spotting for the light.
What can you do? You should repair your windows immediately. It doesn’t have to be a very complicated procedure. If you have a problem with rot, you need to start by digging out the rot, as well as any healthy wood that was in direct contact with it. If not, you run the risk of it coming back again. To fill the hole back up again, you can use any wood filler that is commercially available, sand it smooth and then paint it to match.