Randall writes from Cheboygan, Michigan: "I have 2x4 exterior walls and want to know the amount of long term savings that R-15 will provide over R-13. I have Nat Gas furnace and live in northern MI."

Answer: It is difficult to provide accurate numbers on payback because there are so many variables that come into play that impact energy savings. That said, we recently did a quick analysis with our Global Energy Master® program for the annual savings for 2,000 square feet of wall area going from R-13 to R 19 and found it to be about $85. We would expect the savings in your example going from R-13 to R-15 to be less because the increased R-value is less.

If adding a more efficient insulation to the walls will require opening up the walls to install the insulation, you may find that your time and money are better spent in another part of the home. For example, increasing your attic insulation from R-19 to R-38 can typically pay for itself in about 5 years. There was a study done years ago that showed going from no insulation in a basement to just R-11 can save $400 to $500 per season in cold climates like northern Michigan.

It is hard to put a price tag on the benefits to the environment in terms of reduced use of natural resources and carbon emissions but they are important as well. And then there is comfort: In a well design/constructed energy-efficient home, every room will be within three degrees of the set point on the furnace, including bonus rooms.