[Griff]

Further thought: Many buildings have retro-fitted to make them more energy efficient. This often was accomplished by stuffing the attic full of insulation, sealing air leakage, and screwing up the airflow of the house. This keeps warm moist air in the building. One worry is that the warm moist air will condense on any cool surface, in this case your soffits (hopefully not in your attic insulation). You can grow some interesting fungi this way. They can be harmful, although not generally as bad as the paranoia that was in the press a while back, unless you have an allergy.