+1 what lowCO2design has posted and I comment as follows -
I could feel the heat on the underside of the membrane so this is why I think this is the cause.
Warm moist sir generated during the day lodging in an attic space condenses on coldest roof surface as the structure cools.
We have a water tank in the attic too so could the water in it be heating due to the sun and causing vapour?
No, not unless you have the tank situated directly under a rooflight where the sun could get at it.
Temperature increases due to the attic generally warming up could certainly contribute to the rate of evaporation from the tank.
I think perhaps the tank could have a lid fitted and insulated - it is already insulated around the outside but not underneath.
An insulated cover and sides will assist.
The cover will help prevent evaporation into the attic
No insulation below the tank will keep the water from freezing.
The side insulation should run into the "floor" insulation - no gap.
When I say the soffits are concrete or something similar I mean they are solid and have no vents and it would be impossible to install vents in the soffit so I think extra tile vents will need to be fitted in the slope of the roof.
You could possibly install vents in the roof slope just above the level of the quilted insulation (so they are not blocked).
However you need to get a calculation done to ensure the venting is adequate.
You can drill through most normal construction materials.
Someone should inspect before you do any work.
Would it be sufficient to install 2 in the back or would they be needed in the front also? We only insulated the attic a few years ago and did not have this problem with the uninsulated attic. However the house is much warmer with the insulation -just a pity it has given us a new problem.
You seem to have super insulated the attic without installing a vapour check to reduce moisture migrating through the insulation into the space.
The effect of this has been exacerbated by the relative lack of ventilation overall and in particular the lack of low-and-high level vent arrangement that promotes airflow to carry away moisture from surfaces.
ONQ
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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.