Is it smart to lay plastic under a pier and beam cottage and should I close it in?
Some mold smells exist in our 80 old cottage. If I use the cottage 3 seasons only is it better to lay plastic down and not close in the crawl space in? On the other hand if I used it some times in the winter should I close the crawl space in the best I can and use an exhaust fan with a humidstat control to create some negative pressure?
Anytime a wooden building of any kind – a shed, bunkhouse, cottage or anything – is close to the ground, it is smart to have a moisture barrier on the ground to prevent mold and rot. So whether or not you do a crawlspace underneath, definitely lay down a poly membrane.
If its open air underneath and not used as an air barrier or radon gas barrier, I would cut some slits in it at any low points just on the off chance any bulk water did accumulate that it could drain through. A few cuts in it won’t have much effect on its ability to prevent moisture evaporation from the ground.
Some things to consider whether or not you close it in –
If you really don’t go often in winter then it may not be worth the cost and effort of jacking it up to build a 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 building for comfort and efficiency. You most likely already heat it on occasion, so I think you may be wise to also at least insulate between the floor joists if you just go for the vapour barrier on the dirt solution. That will save you energy and make you much more comfortable.