In addition to installing a paper or plastic moisture barrier behind cement board, manufacturers recommend applying a waterproof membrane directly to the surface of the cement board. This membrane is applied before tiling and effectively adds another layer of water protection in wet areas.
So they are saying use a moisture barrier behind the cement board as well as another moisture barrier on top of the cement board. So you would essentially be sandwiching the cement board.
1) Is it recommended to have a waterproof membrane below and on top of the cement board to sandwich it? If Kerdi was applied to the top of the cement board, then I was told you don't need any moisture barrier at all behind the cement board? True?
2) If it is NOT recommended, then what would be the negative impact of sandwiching the cement board with a waterproof membrane?