Argh, we've gone round on this many times. NexGen has perlite pellets. It was confused by some with their shower floor material which had the styrofoam pellets. Unfortunately it sounds like some folks at USG don't read their own product data. http://www.hearth.com/talk/threads/original-type-durock.69074/#post-868793 Also note greythorn's as hoc test that follows in the thread.