•Cold air is heavier than warm air. When you open the door, the cold air falls out and pulls in warm air. This is noticeable when you stand in front of a refrigerator and open the door with no shoes or socks on. You can feel the cold air falling over your feet.
•Convection demands that temperatures equalize. So the warm air is drawn to the colder areas and visa versa. So when the door opens the warm air naturally migrates to the colder conditioned area. This means the cold air must exit the room at the same rate.
