We're adding a shower to a half bath and during demo discovered drywall has been glued to the concrete block wall. Four feet of this wall will be used for tile in shower. Our Tiler says he can use Kerdi waterproofing but the inspector says we have to use furring strips over the drywall and 1/2 inch concrete board over that. Then tile. My question(s): 1) Do we need to install some kind of vapor barrier in this scenario? 2) Can we use insulation board between fur strips and if so, foil facing tile or away from tile? 3) Any other suggestion to do this per code?

Thanks!
DParker