I'm renovating a bathroom in my apartment and would like to fit 12mm Travertine tiles in a bath/shower area. For most of the walls in this area I have used cement board (Hardibacker 500) attached directly to suitably spaced studding. However, I have one wall which is a boundary between apartments and I'm not allowed to strip it back to the stud to re-clad it.
By drilling a couple of holes I've found that this wall seems to be constructed from double thickness (~30mm) of Gyproc Soundbloc and is painted. I had planned to remove the paint, screw and glue some 6mm cement board (Hardibacker 250) to the wall, and then seal all the edges with silicone. However, I'm worried that moisture could still permeate through the backer board and gradually weaken the plasterboard.
How I can prepare this wall for reasonable heavy tiles, taking into consideration that it'll be a wet area?