waterproofing cement or villaboard

I am going to replace the tiles and bath in my bathroom which has an above the bath type shower. The original wall is cement render over concrete bricks the wall seems quite dry.
I was going to put villaboard over it anyway to make my tiles easier to lay. I have read on the hardie site that the villa board should be battened off the wall and then waterproofed.
I am a bit worried that with battens then villa board then waterproofing then glue and tiles I am going to end up losing a few inches of my small bathroom also the bath will probably be no longer set into the wall by much. If I have to waterproof with a membrane anyway couldn't I just do it over the original cement or even ordinary cement sheets? remembering it is only the top half of a shower wall not a total shower cubicle. is it really a bad idea to screw villaboard direct to the wall?
how can I tell if there is dampness in the original wall? under the bath is completely dry


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