Chris writes yet again from State College, Pennsylvania: "One last question - it is possible to use faced insulation with existing insulation. If so how - face up or down?"

Answer: Perhaps. When insulating on top of existing insulation we recommend the use of unfaced insulation so no vapor retarder will be on top of the original insulation. This will avoid creating a situation where moisture is trapped between two vapor retarders. Having said that I will also admit 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. 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.