I'll shout my response to you if I can manage to get my doorwall open. It's all iced up again this year. The rooms closest to the furnace are no problem but the further you go from the furnace, the worse it gets. And, yes, I do try to balance the vent flaps. Also, the kitchen and bath sills, along with the door near the basement, get iced up occasionally also. One thing that has helped a lot is a make-up air vent to the furnace for outside air. The clear poly will show damage more than paint.