I'm not a building expert, but I do know that when one side is warm and the other side is cold, and wherever between the two the temperature reaches the "dew point", the moisture in the air condenses out on the surface. I think you need a vapor barrier but it might not be so easy to retrofit into an already built structure. Sounds to me like you might have to remove the plasterboard, add a vapor barrier, put up new board. Another possibility is add a vapor barrier to the plasterboard and then cover it with another layer ...but I'm not sure that is good building practice.
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