Cement or high density polystyrene cement faced backer board?

[OuterSpaceHopper]

Hi folks,

I need to refit and tile my bathroom.
The plaster under the old tile is blown in places so I will have to strip it off.

The walls underneath are not very flat so I think I'm going to use a backer board under the wall tiles(20x25x6mm).

The wall is an unusual low density cinder block at about 40m thick, all stood directly on the floor boards.

I've been looking at polystyrene cement faced backer boards, mainly to keep extra weight of the wall down, but they seem quite flexible, and might not be so good to tile onto, or easy to fix flat?

Has anybody tried them?

Or does anyone think I should ignore the weight(after all the wall will be lighter without the old basecoat plaster) and just go for the tried and trusted 12mm hardibacker cement board?

What should I use outside the bath area? is ordinary plaster board?

Any opinions, or fresh ideas appreciated!

Thanks

 

[OuterSpaceHopper]

I suppose everyone knows what hardibacker is.
i.e. solid cement board

http://www.diy.com/departments/hard...tile-stone-l1200mm-w800mm-t12mm/136777_BQ.prd

here is an example of the kind of expanded high density polystyrene\fiberglass\cement face board I am thinking of using.

warmup insulating tile backer board for walls and floors.
http://www.toppstiles.co.uk/tprod4423/section41/warmup-insulating-board-6mm.html