The fundamental mistake is that it was done halfway. Either completely or not at all. Not at all means that it is a true cold floor without any insulation, as ours is. It is dry, airy, ice cold, at least for now. It constantly gets fresh air through the roof soffit gaps and through the tiles. In addition, it has a Velux with a ventilation setting. Completely would mean insulating and heating this room so that it could be lived in. Like other living spaces, it would need to be ventilated. Because modern houses are simply as airtight as plastic bags. That would also work. Halfway means what happened here. Insulated, wrapped up, sealed, but neither heated nor ventilated. So there will be cold air standing there and condensing on the panes, etc. The remedy is forced ventilation. So, trickle vents in the windows, plus mounting a Helios exhaust fan that extracts air up there, which is sucked in through the trickle vents. And depending on the weather, there will also be a draft. Karsten