Mark writes again from West Springfield, Massachusetts: "In my cold space in back of my knee wall in attic are 2X8 (71/4") floor joists. I am going to seal up behind the knee wall as I am finishing the attic space so it is air tight. Since I am having a hard time finding R25 in my area, I was planning to use R-13 faced on the bottom layer (Vapor facing down to my 1st floor) then adding another layer of R-13 unfaced on top of that within each bay then replacing the flooring. Or would it be better to use R-30 faced in each bay and don't put the flooring back so not to squeeze the insulation."

Answer: If you put R-30 insulation in there you will end up saving more over time. Also, compressing insulation is not recommended because you will lose R value. If you did compress the insulation in the situation you describe, you would end up with roughly R-25. It is actually recommended that you have ventilation in an attic that is not conditioned (heated and/or cooled). This will prevent moisture from becoming trapped in an air-tight space.