A client bought a house two years ago with no mold growth in the attic space. He installed replacement windows and added insulation to the attic and basement. The house was warmer but it also retained more water vapor. Since the client did not add ventilation to the attic space to remove the excess water vapor that was created when the house was tightened up, water droplets formed on the attic sheathing (plywood) and allowed the ever-present mold spores floating in the air to start growing. The client created his own mold problem.

I see this time and time again and it is almost always as a result of weatherization programs. The golden rule is, if you insulate, you must ventilate.

wet attic sheathing