The air mass flowing into the door region is characterised by three physical variables.
1. As a consequence of thermal laws, the cold air directed to move inwards mixes with the warm air inside the room.
2. The wind pressure directed at the door area – and therefore at the room interior – must be taken into account.
3. In the entry area, the higher the proportion of cold air flowing inwards is compared with the proportion of warm air flowing outwards, the more power is needed to keep the warm/cold shielding constant.