I am attempting to insulate a catherdral ceiling. While I know that closed cell insulation has a higher R value, the space being insulated has very limited space between the roof deck and the sheetrock, thereby forcing me to deal with the problem of heat transference. I've read that if I don't have enough thickness for the closed cell insulation, the added gain by the higher R value with closed cell will be lost due to this principle. If this is true, what is the minimum thickness i need? I have 5 inches to work with.