I re-read my Follow-ups and I don't see where I said: "... the cold floor was drawing moisture from the ground under the plastic ..." If you re-read the Follow-ups, all I was saying was that warm air holds MORE moisture than cool air, and when the warm air cools, the moister in it condenses OUT onto the joists and/or the TOP of the plastic. There's no way that moisture can get 'through' the plastic sheeting (unless it was a sieve.) If the crawl is vented and it's kinda warm every Winter (as you just menioned above), remember, as warm air cools, the moisture in it condenses out. Hence, the reason you get mold/mildew on your joists AND moisture on TOP of your plastic. Plez keep this principle in mind.
In short, warm air rises. If the rising air is 'full' of moisture from WHATEVER source (be it from the outside air or from any 'exposed' ground in the crawl), it will rise up through the floor and into the room(s) above. And as long as there's an engine (warm air), moisture (if present) will be taken UP with it. If the room above is closed off, the moisture will collect on any cool/cold surfaces that are COOLER/COLDER than the air in the room. Again, when warm air (which has moisture in it) will condense out on cooler/colder surfaces. I'm SURE that if you close up the room in question, 1 of 2 situation is going to be present. 1) The room is going to be REALLY warm and humid, or 2) it's going to be cool in which case the moisture will condense out.
You should re-read my 'Very Good' Follow-up again. In it, I think I have some info for you. What you wrote in this most recent Post, you've 'confused' what I said. If it's better for you, start by reading your original Post and the Follow-ups. That way, everything is 'fresh' in your mind as to what I know (based on what you've told me), and what you need to look into. If you're still confused w/my Follow-ups, just restate your question. (Just be sure that you don't mis-state what I say. Re-read ...) I patiently await.