Insulated tile board for shower

Hi all

I'm putting in a new bathroom suite at the moment and have removed all the old tiles on a stud wall and also an outside wall. (The plasterboard came with it)
With the stud wall I would normally fill it with insulation and then cement board over it for tiling.

With the outside wall I'm not sure what to do about insulation. It's a cavity wall about 10 years old so has minimal cellotex in it and the bathroom is quite cold in the winter, and boiling in the summer!
What's the best way to insulate this ready for tiling? I don't mind losing some space as it's only where the shower is going. I have attached a picture.
I have seen there is plasterboard backed with insulation but none of this seems to be moisture resistant for bathrooms so don't want to risk it, especially as it's where the shower is!

Regards
Mathew

Same treatment as stud work unless you are using shower panels , then cement board not required .

Try this
https://www.wedi.de/en/products/building-systems/building-board/building-board/