Condensation on walls

Hi I live in a coastal city in South Africa that experiences mostly humid conditions and with winters of around 14 degrees C, there is still a bit of humidity. Background: I have converted a dwelling space (6m x 3m) attached to the back of my house into a flat, it has a flat/sloped zinc roof. I installed new ceilings and placed isotherm (insulation) between the ceiling and the the zinc roof. No vents in the ceiling. There is a cornice around the ceiling except for one 3metre part where I will be installing cupboards. Problem: Ever since winter has started about 3 weeks ago, I've started to get what appears to be condensation appearing on the interior walls, quite wet that when I run my hand over the walls it drips. Question: - Could this condensation be caused by the zinc roof? - Is it possible that the isotherm (insulation) that I installed is interfering with temperatures inside the roof as there is no open air space between the zinc and insulation? - Could the open gap of 3m without cornice be allowing air up into the roof ceiling creating some form of temperature change? I've tried leaving all windows open to allow air flow in the room but it still persists, PICTURE: this is what the roof looked like before I did all the work.