I’ve looked into this quite a bit and decided to go for premtool 12mm tile backer boards. I worked out the following;
prices inclusive of vat
Plasterboard: £4.5/m2
paintable tanking £10/m2
impey water guard £21/m2
hardie backer: £13/m2
Tile backer: £12.5/m2
both the PB and the hardie backer should be tanked, the tile backer is already waterproof and just needs the joints sealing with CT1 or similar.
i therefore believe, for a tanked wetroom, tile backer board is the cheapest option.
furthermore, plasterboard can only take up to 32kg/m2 (or thereabouts) which is right on the limit for a 10mm marble, the tile backer board is good for 120kg/m2 and the cement board 200kg/m2.
I could have gone cement board (I did at the last place), but they’re heavier, messier, and still need tanking, therefore slower to install and more expensive.
modern xps filled tile backer board seemed to me like a no brainer.
I liked the look of the impey waterguard self adhesive waterproof sheet (and decoupling membrane), but so expensive, the only people that use this must be limited by buildup height.
Ive gone tile backer wall and floor onto 22mm p5 caberdeck