This is pretty neat. The roof deck is a couple of degrees colder than the outside air temperature due to night sky radiation. This also drops the temperature due to night sky radiation. This also drops the temperature of the top surface of the attic insulation. The outside dew point is very close to the outside air temperature during the winter. When exterior air is brought into a well-insulated attic assembly, the dew point of this air is sometimes above the surface temperature of the underside of the roof deck and the top surface of the attic insulation.