Tub surround Waterproofing, I'm so confused
DH is tiling new tub surround. Tub has been in place since 1952 and walls are not all open and not all going to be opened up, so sheet poly behind board does not reach to floor, only to tub. He can't get behind and beside the tub with it to do a proper vapor barrier. It is stapled over most of the studs but open at top and bottom.
Board is Aqua Tough 1/2" fiber rock tile backer board.
We keep seeing Kerdi on TV watching Holmes on Homes but I keep reading that Kerdi would be more expensive than using Redgard or Hydroban especially for a tub surround not steam shower.
There are two niches. Kerdi seems like it would be a PITA.
Home Depot doesn't carry Redgard, Kerdi or Hydroban. No idea where to buy or which one to use and how much to buy. Price is a consideration for sure. DH is very handy, is a HVAC installer for a living. Has done tile but not a shower.
He is planning on using Ditra on the floor. They do carry Ditra.