Andrew Gabriel wrote:

Now, the increased ventilation means more outside air is passing through
the loft. The increased insulation means the loft isn't being warmed.
On cold clear nights, exposed surfaces such as a roof radiate heat and
in the absence of any heat source, cool below the outside air temperature.
This will now drop below the dew point of the outside air, and form
dew/condensation on the inside of the roof, in the same situation you
get dew outside.

Does this sound plausible?


Yes but...

Any bright ideas on fixing it, apart from rolling up the loft insulation
and blocking off the new air vents? It seems to me such a roof is
dependent on heat leaking from the house.


Do you have loft mounted tanks that are un-pressurised?

I have a modern roof that is precisely similar, but there is no damp at all.

BUT there is almost a hermetic seal from downstairs.. and a few heats
sources in the shape of loft mounted lighting transformers, that
overheated in the insulation, that now lie on top of the fully boarded
structure.

open header tanks that are kept well above freeing can be a huge source
of moisture.