I'm adding insulation to the stud walls in my baserment with 2.25" of close cell spray foam on the concrete walls. The stud walls are 3-5" " from the close cell spray foam. I can either (try to) pack the void between the wall and the uneven surface of the closed cell spray foam, leaving the stud bays empty, or leave the air gap beween the stud walls and install the rockwool betweent the studs as its designed to be installed. Its a pretty easy to get a good result placing the rockwood beween the studs. Filling the air gap is a bit more.of a challenge given the uneven surfrace and fact that the distance between spray foam and inside of studs is inconsistant. Moreover, 16" on center studs and 15.25 wide rockwool require either leaving gaps behind the studs or a pretty poor looking and performing insulation job.
My concern with leaving a large, contiguous air gap is that mold has potential to grow between the stud walls with the rockwool insulation and the closed cell spray foam. My concern with trying to fill the void will also likely have air gaps and the rockwool gets pretty beaten up when stuffing it between the studs and the uneven closed cell foam. Morevoer, the alighment with the rockwool and the studs quickly goes out of alignment leaving air gaps no matter how well you snug them up to each other.