The plastic MUST be removed from between the two halves. That may well be the cause of your problem. Failure to remove the plastic creates severe condensation issues in cold weather. While the marriage walls are not insulated, there MUST be insulation placed between the marriage walls at the ceiling line and the end walls plus at the floor which you saw from the basement. Again, failing to do this creates severe condensation in cold weather. These things are not optional, they are essential with every home.