Frank writes from Phil Campbell, Alabama: "I have a 5" blown insulation attic at present. I purchased a kraft backed 40 rolls and began to put them down. When I went to your site it stated that if you already have blown insulation to use un-backed rolls. Is there a major problem with using the backed rolls I have purchased?"

Answer: Probably not. You are correct in understanding that we recommend unfaced insulation when insulating on top of existing insulation so there will not be a vapor retarder on top of the original insulation. This will avoid creating a situation where moisture is trapped between two vapor retarders. That said, I can also report that a vapor retarder can be placed up to one-third of the total thermal resistance in the assembly starting from the warm side. If the insulation you are adding has at least twice the R-value of the insulation in place, then it should work. Since you have only five inches of blown insulation at present, the insulation you are adding will probably have at least twice the current R-value. Unless you live in a very humid area and your local building code has a special requirement, the vapor retarder should always be on the warm-in-winter side, which in this case means down on the existing insulation.