Whilst i agree fully that the ventilation should be checked, would you not agree that what is happening now is flagging that there is a problem with the insulation levels?? Increased ventilation may solve the condensation forming on the felt, but isnt solving the primary issue of the heat escaping into the attic cavity.
oh, and the extractor fan point you made is excellent.... that could be the source of your water vapour.
1. (primarily) heat rising from underneath.... inefficient insulation
2. stagnant air currents.. inadequate ventilation
3. solar gains on the dark coloured roof finish
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Ok, but I think the issue of insulation is a red herring, whether the ceiling is insulated or not there is a condensation risk in the roof space if there is not adequate ventilation. Simple example: sit in your car on a cold day with windows closed- the windows will quickly fog up as water vapour condenses on the cold glass; if window was left open water would not condense as ventilation ensures that saturation point of air is not reached. The warmer the air the more water vapour it can hold, when temperature drops the air can hold less vapour and condensation is the result. The old maxim: "keep vapour barrier on warm side of insulation" applies.
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