There is nothing that stops the humidity in the attic from turning it into mush tho. The vapor barrier is typically installed on the top of the ceiling drywall and the warm moist air is in the attic in the summer. The dew point here is generally below the temperature of the air in the house and hence the ceiling. Moisture starts condensing on the bottom of the cellulose stack and it turns to a gray mat. I can mail anyone a sample if you like. I have an attic full. It even compromised the R-19 fiberglass under it. I ended off worse than I would have been if the previous owner had just said no to the cellulose salesman.