1.   By attic ceiling I presume you mean insulation in the roof, between the rafters.

2.   Normally insulation in the attic ceiling, between rafters, is installed with an air space between insulation and roof boards.

3.   Pick a spot in the attic ceiling that is close to the fan yet clear of obstructions.

4.   The diagonal slant of the attic ceiling helps guide your eye down from father to mother to the children, and locks the composition into place.

5.   Without such a seal, moist air could escape from the duct before it goes through the ridge vent, resulting in a wet attic ceiling and possible mildew.

6.   Without such a seal, moist air could escape from the duct before it goes through the vent, resulting in a wet attic ceiling and possible mildew.