Why do the building make vapor barrier, why is it needed
The enclosing structures pass water vapor due to steam pressure and movement of air masses.
In winter, there are especially many vapors in the premises, and due to the difference in the internal pressure and pressure of the atmosphere, the pair penetrates into the enclosing material. Most building materials, one way or another, are not steam – and moisture resistant. Exceptions are material such as polyethylene.
The movements of the air masses caused by the winds, as well as their own ventilation of houses, also provokes the penetration of steam into the facing and insulation materials of the houses. The whole point is in the same pressure, when the pressure in the house rises and its indicator becomes higher than outside, the steam rushes out, penetrating all available alkalis.