[jeffnc]

I also hate the way Durock crumbles, but I equally hate having to cut any cementitious board when drywall will do just fine. (And by the way, green board or other such things are a waste of time and money IMO.) Even in showers I rarely use CBs - I put Kerdi over drywall, or I use Kerdi Board.

As for building up a mud floor, "passe" is one word used above. A massive pain in the neck and outdated and unnecessary are others. If you need to raise the level of the floor to match old transition heights, just use plywood. If you need more structural support, again just use plywood. In theory it's probably best if the dyywall is slightly raised off the plywood about a quarter inch. For the floor I would use Ditra, or Ditra XL if you need more height. There are other uncoupling membranes out there made by Laticrete, Mapei, and Custom as well which should perform the same (unless you want waterproofing). Any of them are better choices than CBs, which I basically avoid when I can. If you do use cement boards, I would use the 1/4" Hardi instead of 1/2" Durock.

 

[jeffnc]

Thanks everyone. We wanted cement board simply because it's a bathroom and things can get wet - the old drywall removed did have mold even in areas that were not around the tub/shower. I would definitely feel better with something's that more tolerant of water than drywall.

Of course you would never install directly over drywall. But keep in mind - you never want water to get to your substrate. Cement board is water resistant, not waterproof. That means if it gets wet, it won't itself get damaged, but it will soak up water like a sponge, it will stay wet a long time, and it will rot whatever it touches (e.g. studs). So you need waterproofing not matter what. Personally I prefer Kerdi membrane over drywall. Drywall is much easier to work with than cement board, and that is a completely waterproof solution. You can of course put Kerdi over cement board, but why bother with that hassle? Even a liquid applied membrane such as Aqua Defense can go over drywall. (I haven't used Redgard and it has a truly senseless spec for drywall so I won't go there.) Keep in mind - if any water gets past your membrane, you've lost regardless of what your substrate is.

 

[jeffnc]

The only issue I have with Hardiebacker is that it is thinner than drywall. Hardiebacker is only .42" so if you are trying to match it to drywall you need to build it up more.

Well... not necessarily. If you put the Hardi/drywall transition where you want the tile to stop, this shouldn't be an issue. You're talking about in the neighborhood of 1/16", and thinset will make up at least that much. In any case, it might actually look good to have the tile stand a little less proud on the wall.