My house is a two-apartment house, where I live on the first floor and rent the second. It's an early 1900s plaster & lathe, and currently has no insulation. I want to add rockwool in the ceiling of the first floor to both insulate and sound-proof. Since each apartment has independent heating (ie, each has it's own steam heat boiler), do I need a vapor barrier on the bottom-side (first-floor facing) of the rockwool? If so, would that be 6mil plastic? I will be replacing the ceiling plaster with drywall, and I prefer rockwool for both it's density and fire safety.