The crawl space in my new house (new to me) ranges from 3 to 5 feet in height I believe it is built on a Rock ledge. The insulation on the ceiling has black spots (mildew ?) and is falling down. When I replace the insulation should I put a vapor barrier on the ceiling? Also, there are open vents down there. Should I keep them open year round? I'm in New York An i'm sure it will get cold down there in the Winter.