Hi Trip,
Breathable membranes are a case of, if it works, its fine, but you may find you cannot show prima fascie compliance in relation to ventilation if it doesn't work if you haven't left the 50mm gap. This can result in remedial works and problems at sell on.
If you're determined to use a breathable membrane, you will need to check if it has an agrement certificate and ensure the works comply with the specification thereon - for example the installation of a vapour check or variable vapour membrane. If it still fails I don't know what you can rely on to finance a remedy unless you get a guarantee that's enforceable.
Incidentally I've seen several websites dealing with attic conversions and roofing insulation packing the rafters the gills with rockwool or quilt. They have no clue what they are at. Insulation works by trapping air. Compressing it doesn't trap MORE air, but less, since you now have more insulation fibres in a given volum than normal.
ONQ
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