1.   After sealing the joints, wrap the ducts with foil-faced fiberglass insulation.

2.   I have considered wrapping the ducts with reflective insulation, or stapling reflecting foil on the roof rafters.

3.   If the duct is hanging down from the joists, and there is space between joist and duct, you can wrap the duct easily.

4.   Wrap the ducts with this insulation, vinyl side showing, and tape all joints.