This is a new one for me. Got a call from a client wanting some water damage repaired from a leak (caused by damage to masonry joints during a window retrofit). I opened a hole in the interior sheetrock and pulled back the damp fiberglass roll insulation, expecting to find tar paper or Tyvek between the studs and brick, but instead found another layer of sheetrock with a brown color. Research showed me "exterior gypsum sheathing" which seems crazy to me, knowing how sensitive any wall board is to water.
This stuff is soaked and crumbling, already completely failed in the spot I checked, with about a 1' x 1' hole open to the back of the brick. The sheet is attached to the studs from the brick side, with only 1" or so of air space, so not sure how to even get in there to either replace it with something better, or close it back up.
Has anyone else ever encountered this problem?