I am asking for advice on thermal insulation of the attic with mineral wool, more specifically, I mean the need to leave an expansion space between the vapor-permeable foil 1000 mg / m2 and mineral wool. W 2010 year I started construction, while buying materials for building a house, I bought a metal tile, foil with permeability 1000 mg / m2, having previously asked whether an expansion space will be required for this foil; the seller assured me, that there is no such necessity. After the roof modification work has been completed, I started insulating the attic; the rafters have a height 15 cm, that's why I bought wool 15 cm and a vapor barrier foil. After thermal insulation was made in the winter, I found it, that the wool in direct contact with the vapor-permeable foil is moist, and even wet in some places. Is this symptom normal, or did I make a mistake listening to the seller of this foil and not making dilatation between the foil and the mineral wool?
The symptom found by the Lord is not normal, and the explanation of its causes is also accurate. It was necessary either to use a larger distance between the foil and the thermal insulation, or use a foil with a higher permeability: above 3000 g/m2.