i am installing a shower in my master bathroom. i tore out the old one, and am installing new walls, and shower pan. my brother in law (licensed plumber) installed the shower pan. i bought the durock cement board and am installing that right now, to tile later. my question is do i need to put a vapor barrier up behind the cement board, or can i just tile to the cement board, and seal the grout, and 1/4" gap between the pan and board??