With the attic snug it was important to control any dampness from the basement. If the dampness isn't controlled it will rise and create mold as the moisture has nowhere to escape (since the attic is sealed). Also the windows will frost over every time it's cold outside. So a vapor barrier is needed!
The plastic is 6 mil thick and the guys from Potential Energy did an amazing job sealing the floor. Within two days it was noticeably drier in the basement.