Damp on cold wall

[jimmy1999]

I have a damp problem in one room with mould growing on the two external walls. I believe this is due to condensation although i dont get any condensation on the windows (they are double glazed).

My problem is i dont really no what to do next. I have removed all of the wallpaper that had mould and am putting in ventilation into the chimney. But i dont no how to finish the walls.

I have read about using thin layers of polystyrene on the walls to prevent the mould or should i just plaster it. Any advice on how to stop the mould coming back and if this does sound like condensation would be much appreciated.

Thanks for your help