We put the plywood onto the inside of our stud frame back in June/July - all seemed fine. On Friday we returned to the workshop after a fortnight away and found every wall covered in black mildew. There has been plenty of airflow in the workshop as it has no front doors as yet. Does anybody have any ideas about why this has occurred? It has been quite stormy recently (SE Cornwall), but only normal for Cornwall, and the walls have stayed dry as the building is wrapped in Tyvek.   There