I wouldn't spray or lay anything on the TOP of the ceiling that will trap moisture. In short, you want the moisture to rise and 'leave' the house. If you can insulate the ceiling well you shouldn't have any problem with the moisture getting out. It will move slooooowly when it does because, well, insulation is dense and INTENDED to 'trap' air (hence, moisture too.) BUT, it will get out.
Forget laying anything like plastic sheeting. You'll certainly CREATE a moisture problem in doing so. My best to ya and hope this helps