the condensation will only be caused by water/damp which is already there, insulation wont help. It wants drying out completely, know anyone in the bodyshop trade? ideally wants putting in a paint oven for an hour or so.
Then try and locate the water leak which im assuming it has somewhere, many of the mass produced convertables suffer with water ingress,basically poorly fitting roofs/seals etc. its not just Vauxhalls , Fords & Peugeots have had issues in the past. It gets worse as the vehicles get older and the rubber seals perish or wear...