I have installed a bathub and plan on tiling the surround with 4" tile and 1/8" grout. I already have the hardibacker in and I did it without any kind of membrane. I did it this way because the several people I spoke with say it's unecessary. Now I'm worried I should have done it and am considering taking out the hardibacker to put the membrane in. Maybe I can get away with skipping the back wall? The problem is I already have the adjacent walls nice, spackled and painted.. It's seriously going to set me back but I'll do it if it's a must.