White mould in loft space?

Hi, hope someone can help. I moved into a house which has a loft space plasterboared and was already being used as a small home recording studio. I took advantage of this being an amateur musician and added a stud wall for the neighbours benefit regarding noise. Trouble I have now discovered is that i started to get condensation and it has now caused white mould spores to appear. The room is not constantly heated as to cost of fuel etc and only gets used for a few hours rather than all day. There is one Velux in the room but no other way of ventilating the room. I didn't want to start adding vents as wished to keep the room as warm as possile but am I creating problems rather than fixing them? Do I need a steady source of heat or vents? The room gets very cold in winter and very warm in summer so suggests the roof isn't insulated. Help!!!