If you have to spend $10,000 to save $500 a year it will take you a long time to break even. If you spend $100 to save $200 a year, you make your money back before the first winter is over and the rest is cash in your pocket. Cost effectiveness is the bottom line: where can each dollar be most effectively spent? and at what point is it cheaper to simply heat the house than to try and save more energy?
These are not always simple questions. You must balance many complex and interdependent elements. The discussion on "insulating objectives" in the first part of this book will give you some general ideas about common results of these cost effectiveness calculations. The most important thing to keep in mind is: A house is a complete environmental system and you should build a budget that will rewinterize this house, not simply reinsulate it.