My Van is reasonably insulated; however I sometimes find myself in zero temperatures in the Northeast. When this happens my furnace runs constantly. I have a curtain of warm window material separating the cab from the cabin that helps a great deal - although it was pain to make. I have two sleeping bags that I was thinking of hanging on the rear doors to add additional insulation. I also have a set of blackout curtains that I could hang instead of the sleeping bags. It seems obvious that the sleeping bags would be better...but I thought I would seek the wisdom of the group.