Have you ever had a home energy audit? You might have paid a professional to do it, or followed a do-it-yourself plan that you found online. Either case, the result might very well have told you about shortcomings in your insulation. The most likely problem in many homes is the area around the windows. Leaking windows can cost you hundreds of dollars every year, but it is definitely a problem that can be fixed. Try to find where there are cracks and holes around your window allowing the air to escape. Using a thermal imaging tool can help you with this, especially since it can pinpoint with great accuracy the temperature differences around the window.
Once you have identified the problem areas, you should take your time and do a good job, especially if you are dealing with wood rot. Leaving even a tiny trace of wood rot behind will make it much more likely that the problem comes back in the near future.