I'm reboarding an old bedroom wall and found some mineral wool batts already in there. Planning to add more but I'm not sure about the membrane side - do I need to face it a particular way or add a separate barrier?
You don't necessarily need a vapor barrier under mineral wool since the batts themselves are vapor-permeable, but you should check your local building code and climate zone - cold climates often require one on the warm side of the insulation to prevent condensation issues, while humid climates might need it differently or not at all. If there's already a membrane on the existing batts, face any new ones the same way for consistency.
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