John writes from Manchester, Tennessee: "I currently have 9" of blown in cellulose attic insulation in my attic, what is the maximum r-value of insulation I can add for maximum energy efficiency?"

Answer: There is no maximum amount of insulation for energy efficiency but insulation thickness does eventually reach a point where it takes a long time to save the cost of the additional insulation. For many areas of the country the U.S. Department of Energy recommends up to R 49. You can find a summary of the recommendations at the Energy Star website on the page for Recommended Levels of Insulation. I don't know the R-value of your current insulation because it may have settled since it was installed. If it hasn't settled much, it may have as much as R-33. To achieve R-49, you will need to add about R-16 with either more blown insulation or insulating batts.