Firstly some background on our project on the west coast of Scotland. The structure is timber framed with attic trusses built within the solid walls of a former farm building. We are installing underfloor heating on the ground floor with radiators in the first floor attic rooms. A mechanical heat recovery ventilation system is also being installed and the insulation and airtightness are to a reasonably high standard. The flat ceiling of attic rooms has 100mm glass fibre between the ceiling joists with 150mm Kingspan above. There is good ventilation from facia vents, through a 50mm gap behind the 200mm sloping coomb insulation to the cold loft space which is vented at the ridge.
One school of thought says to fully fit a vapour barrier between the plasterboard and insulation in all plasterboard surfaces of the attic rooms. I.e. Vapour barrier on vertical coombs, sloping coombs and flat ceiling in order to prevent moisture entering the insulation layers.
The other viewpoint says one should fit a vapour barrier to the vertical and sloping coomb surfaces only so that moisture can escape through the ceiling to dissipate in the ventilated loft space.
Which approach is better in our circumstances?
Thanks for your help.