Bill writes from Malvern, Pennsylvania: "I want to add insulation to my attic. I have 2 by 6 joists, in which R-11 insulation has been placed. I plan to add R-30 on top. Should I place the R-30 perpendicular to the joists (which would leave a 2" air gap, as the R-11 of course doesn't come to the top of the joists) or should I place the R-30 directly on top of the existing R-11 (i.e., in the joist cavity)."

Answer: The best application would be to bring the insulation up to the level of the joist and then go perpendicular with a second layer. For example, install R-11 and then R-19 over the top. This will help to insulate over the framing. An air space between the layers will reduce the effectiveness of the insulation. I understand that installing two layers requires more labor so if you still only want to install one layer of R-30 it should be placed between the joists.