I'm attempting to tile a tub shower surround. I have completely removed all drywall on the walls around the tub except for from ceiling to about 2 ft down the wall. The existing drywall plus the plaster is 3/4" thick. The cement board is 1/2". Can I place 1/4" Luan behind this 1/2" cementboard to make up the 1/4" depth? I was told at my local home store that this is ok but I'm concerned about moisture between the Luan and stud walls. Note: this is for inside walls, no walls are outside perimeter walls. If I use the home stores recommended method do I use a roofing felt or plastic barrier behind the Luan? Is there a better way to make up for this depth issue without removing all existing plaster and drywall?