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As a moisture guard for the shower wall, which does better job, cement board or Shulter Kerdi? Or do you use both?
Cement board is not waterproof. In that water passes right through it like a sieve.
So a vapor barrier is required. It can be tarpaper on the studs, or Redgard or another liquid waterproofer. Or Kerdi. Kerdi applies OK to Durock, not so easily to Hardibacker. Kerdi will also waterproof regular drywall if you want to save steps.