In houses it is ever worse, because additional moisture is generated from cooking, bathing, washing, and other water-related activities.
Amazingly, this moisture can seep through the walls and into the insulation, wreaking havoc with mold, mildew, and other problems. The same thing happens with attic insulation.
You will need to place some type of vapor barrier between the insulation and the living area. In other words: at the bottom of the batt insulation. If this sounds counter-intuitive, think of those times that you may have tacked up batt fiberglass insulation in your walls. Didn't you tack it up with the paper side facing your living area?
Even though you cannot access the bottom of your attic joists/rafters, the insulation still must be laid "upside-down" with the Kraft paper facing downward to the living area.
With loose blown insulation, it's the same story. Lay down the polyethylene vapor barrier first, then blow in the insulation.
Note: We have not found much information to support the idea that Roxul insulation doesn't need a vapor barrier. Literature says that it "does not promote rot," though this is about all we could find at this time. We have contacted Roxul and will update when there is a reply.