Why does one have to leave and 1/8 inch from the new tub and the wall and what
Material do you use behind the cement board that surrounds the tub. Should it be
Felt paper or a plastic poly material as a vapor barrier?

Why does one have to leave and 1/8 inch from the new tub and the wall and what
material do you use behind the cement board that surrounds the tub. Should it be
felt paper or a plastic poly material as a vapor barrier?
Plastic poly is a better vapor barrier. I've never heard of 1/8th inch between tub and wall. There should be some space between tub and cement board. Say 1/8th inch for caulking.