Jay J is exactly right. You have to give moisture a way out of your house and the attic is the place that this happens. The sleeved fiberglass batts do not act as a vapor barrier and the humidity goes right through them up into the attic space where outside ventilation carries it away. If you do not allow for this, you have problems with mold, mildew and window condensation. But you do want a vapor barrier on the 4 outside walls and the basement floor.