I took out my existing shower/tub and am replacing the backerboard. Does anyone know if it is okay to use 1/2" backerboard right over the studs, then placing the tile right on the backerboard using thin set? Some people are telling me to use green board sheetrock over the studs, then use backerboard over the greenboard....do I need to use greenboard at all??

Do I also leave the backerboard 1/4" off the tub and fill that gap with silicon caulk, or do I bring the backerboard all the way down and touching the tub?

Thanks