Can you install insulation behind GP DensGuard tile backer board in the bathroom? On their web site it only says not to use a vapor barrier and nothing about insulation.We are removing the "brick nogging" from in between the studs that Senior Member JLMCDANIEL advised me on.The bricks were the only "insulation" in the walls.If allowed what would be the best insulation for this bathroom? Thanks in advance. Jim