Bob writes from Sunnyvale, California: "I have 2x8 rafters (7.25" space) and need R-30 insulation in my ceiling to meet code. I would like to put a plywood floor down on top of the rafters to make my attic space usable. Can I use 2 layers of R-15 insulation (each 3.5" deep) to achieve the total R-30 insulation? This would leave only 0.25" of clearance between the insulation and the attic plywood floor. Note, the attic ceiling will only be 5'11" high, and I'm trying to prevent losing any walking room by raising the floor."

Answer: As long as the insulation is not compressed, Fiberglas™insulation can be layered and the listed R-values are cumulative. In your case, two R-15 layers will equal R-30. Just be sure that the second layer of R-15 is unfaced to avoid moisture issues.