Luke writes from Portland, Oregon: "I have 2x8 rafters in my ceiling (16" centers) and I'm wondering what the best insulation would be. I'm considering 2 layers of r-15 fiberglass insulation. Would that give me r-30?

Would a layer of 2x6 insulation and a layer of 2x4 insulation give me more r value? I know that I could attach a 2x4 furring strip and put the R-30 2x10 insulation in, but I don't want to do that if possible."

Answer: Yes. Two layers of R-15 uncompressed will give you R-30. Adding R-19 and R-13 insulation without furring out the rafters would compress to approximately R-25.

Furring out with 2x4's would be the proper way to do this. This would allow the proper space for sufficient R-Value.