With fiberglass tub surrounds, the flanges are typically attached to the studs, and moisture-resistant wall finish is overlapped above and sealed. It appears that the LEED prerequisite in question would require 1 layer of backer board full height behind the tub surround, and than additional layer above the tub to cover the flange, am I reading this correctly? What about the "if wallboard is installed" part of the language? What does that mean when it comes to area behind the fiberglass enclosure?

Thanks!