I have a one-piece tub surround. Behind it is the wall framing of course. I'm going to add insulation, but do I need to apply green board? Or I have extra durock as well. My drywaller said to just leave it open but that makes me nervous. I wouldn't mind having the cement board behind there for tub-wall support if nothing else.
The majority of tub surrounds are designed to be installed directly on the rough framing. There are a small number of tub surround manufacturers that stipulate in their installation instructions that a backer (like cement board) is required to be installed, in order to re-inforce the thin wall design of their particular tub surround product.
To clarify: unless your particular tub surround manufacturer stipulates it, there is no need to install a backer behind the tub surround.