Is it smart to lay plastic under a pier and beam cottage and should I close it in?
...crawlspace, but if you do, see here on how to insulate a crawlspace properly to avoid mold and heat loss. And no, we would not recommend creating negative pressure if you close it in. Make it as airtight and moisture-proofed as you can and manage any moisture there is with a dehumidifier.
There is also the matter of bu...