Following current building regulations we have put a 2inch air gap between the roof felt and kingspan insulation. The air gap is vented from the soffit boards at the base and ridge vents at the top. The roof faces south north and on cold still days water regulary condenses on the north facing roof felt. Do we need more of less ventilation? What would happen if we stuck marine ply to the roof felt in order to reduce the temperature difference between the felt and the air?