I live in Northern NM with cold winters, and range 5 per map.
My house was built in 68 the put rock wool batts in r 5 or 10 vapor barrier facing up. Then at some point some laid r15 craft face batts up cross wise over .
So I am now trying to decide what to do.
1 remove rock wool lay down the r 15 against the dry wall, roll R 30 on top of that.
Should be no gap between. I'd use the batt that can fill or overflow the ceiling joists, paper down on the drywall. Then the rest on top but without the vapor barrier or cut slits in the paper without cutting too deep into the batt. Ideally, batts should lose the moisture into the attic then vent it out.