question on where/when to install plastic or tyvek over fiberglass batts.

in an unheated (and seasonally damp) sand floored cellar–we have put up R 19 in the joist bays–the floors above are still wicked cold and there is so much air movement/drafts that I wanted to find a way to limit that. But if we staple up 6 mil or something else, wont that space become a moisture trap/mold farm? I have heard that insulating basement ceilings is a waste, and I have heard that plastic is a no go in humid areas–what to do? but winter is brutal and we don’t want to be heating a seive!