They are polyethylene sheets that are used to prevent interstitial condensation.
The vapour barriers are placed on the hot side of the enclosures, as their function is to prevent steam from passing from the hot side to the cold side, which is the situation in which condensation occurs.
Due to this, in cold climates the sheet is placed inside the insulator, and in warmer climates, on the outside.
There must also be an air chamber inside the enclosure to allow the evacuation of the steam.
It is also used in the screeds, to protect them from the humidity of the ground and to avoid phenomena of efflorescence by capillarity.