The more we insulate our houses, the more risk for moisture problems we encounter. We want to make our constructions airtide and accept the risk of poor ventilation.
Insulation by means of mineral wool is only effective when the wool is encapsulated by an airtide envelope. Otherwise cold air will easely breath through the insulation. Also we want to keep moisture out of our insulating layers, for each percentage of moisture will dramatically lower the insulation value.

What we thus need, is an intelligent envelope around our insulation.

In the northern part of Europe, a very simple rule is prescribed:

Use a vapour barrier on the inside (the warm side) of the construction. Then install the insulation and cover this with a breathable membrane on the outside (the cold side) of the construction.

Often, a simple mono polyethylene plastic layer is used for a vapour barrier. In theory this works well, but in practice the tolerance of the film is way too thin. Also pinnholes and thinner area's in the film can cause leakage of moist.
Meuwissen has 2 classes of reinforced moisture barriers and they cost only a few cents more.