bstop, I have worked on humidity measuring equipment, I need to clarify your statement.
If you heat up cold air it becomes dryer, and is able to absorb more moisture. (the measurement is called (RH or relative humidity) the air must obtain the moisture from somewhere, if not it should dry your attic if it is heated up. The air in the house takes up moisture because of the fact we are living in it, breathing, cooking, washing, etc. If air leakes from the house in to the attic it bring with it moisture and condenses in the cold parts and possibly cause dripping. The attic should be sealed from the house to prevent air entering, convected heat from the house should heat the attic without moisture.
All holes including the attic hatch should be sealed.