Yeah, originally I thought about doing both RedGard and plastic sheeting, but after doing a bit of research it seems that this is not safe. Apparently, if you do both of these moisture barrier methods, vapor can get trapped between the cement board and the plastic, and then it has trouble escaping which can cause issues. I'm not an expert at all, but that was just what I have heard.
Oh, and I don't know if this makes a different, the tub's long side will be up against an exterior wall.
Last edited by amodoko; 01-18-2012 at 04:31 AM.