sz1, hi
What is happening is that, water vapour, from cooking, drying clothes, showers, baths is migrating through the plasterboard and the Insulation into the loft space.
What then happens is that because the warmth can not get into the loft, but water vapour can condensation WILL occur, the heat within the rooms in the house cannot get into the loft space because of the insulation. Water vapour is a molecule air temperature is simply a change in the background, ambient heat in the air.
If the roof void, the loft space is not adequately ventilated then the dampness that has percolated into the loft space simply has to condense on any cold surface.
You would not believe what happens if there is no or limited ventilation a loft space when the Home owner has increased the depth of insulation and the external air temperature drops to well below zero, I have seen the inner surfaces of roofs raining into an attic at temperatures of 10 or 15 below zero? In Scotland a few years ago we had about a week of minus 20 this generated a load of work for me, as policy holders reported leaky roofs, stains on plasterboard ceilings Etc. and it was all down to condensation, proof, no rain.
As previous, do NOT under any circumstances apply the dreaded spray foam.