Ron, that’s great info. What is the IBC section reference?
Section 2509, both editions.
Anne:
Both Durock (USG) and PermaBase (National Gypsum) have 5/8"-thick products.
“Wet walls”:
If memory serves, HUD’s now obsolete Minimum Property Standards required M-R board “in bathrooms and on all wet walls”, which their inspectors and reviewers defined as “anything with a water pipe coming through it”. That meant the kitchen sink wall, and the washer/dryer wall, but not the other walls in those rooms. It also meant only the plumbing wall in a half bath (no tub/shower). Full baths got M-R board on all walls.
This goes back thirty years or so, when all there was, was greenboard. Unlike IBC today, I doubt the building codes mentioned it at all. Of course, good architectural practice called for us to use at least MPS requirements on private residential projects as well.
I think as we trend to paperless gypsum board, traditional M-R board will vanish, and there will be a (if there isn’t already) a Dens-“something” glass-mat product that will work as a tile backer and will be able to be finished like gypsum board. Same board for tile or paint, stop the tile where ever you wish.