Contractor doesn't use vapor barrier behind tiles in tub
After following this forum for awhile, I've noticed that it is strongly recommended to install some sort of plastic sheeting behind the cement backerboard behind tile installations that will be getting wet.
I spoke with my contractor today who said they put up Hardiebacker or Durock cement board, and that it waterproof enough, so they don't put up any other vapor barrier. It sounds like they've been doing it that way for years. But I could sense some hesitation, almost like he was thinking in his head "wow, I wonder if I should have been putting that stuff up?". He said that leaks would usually occur around the shower valve, and plastic sheeting isn't going to help there anyway.
Should I insist on it? What should I do?