I like to see the Wood Moisture Content (WMC) below 16%. Mold does not grow on wood till it has a WMC of about 20%.
But I like a safety margin. I would recommend a remote hygrometer installed in your crawlspace so you can monitor the conditions in the crawlspace. You want the relative humidity 80% or less.
I have found Vinyl Gutter sealant sticks to the plastic sheeting better than anything else. You can buy it at Lower in a caulking tube. It also sticks to masonry. Most adhesives do not stick to plastic sheeting at all. Duct tape does fairly well also.
Remember, Air Conditioning begins with AIR.