A well-designed building shell functions in principle like a down jacket: there is an outer and an inner layer, and between those is the insulation.
Wind-tight level (= the outer layer)
This is intended to protect the building component from rain and wind. Exposure to the wind is a continual strain of friction on the building. Without a wind-tight layer the building is subject to airflow from outside and the thermal insulating properties of the construction are reduced. Entry of moisture can result in structural damage and noise protection is also impaired.
Air-tight level (=the inner layer)
It is important to protect the building from adverse weather conditions. Indoors the room humidity must be prevented from penetrating into the construction and the insulation. The air-tight layer is always on the so-called warm side of the external building elements.