Attic ceiling insulation in a vented attic question – GreenBuildingAdvisor

I’m a little embarassed to ask a question that has arisen since I decided to redo my attic HVAC and insulation.  Background:  I’m going to completely air seal the attic ceiling from the house below.  The HVAC equipment and ducts really have nowhere else to go so I plan to deep bury the ducts.  I’m in CZ 3A.  Complete compliance with the code as I understand it seems nearly impossible because R13 ducts are not available, whereas R8 are readily available.

My question has arisen after considering the consequences of deep burying new R8 ducts (R13 is code but not available) and avoiding condensation on the outer duct jacket.  I understand why no condensation forms on the jacket under the existing conditions (the skin is the same temperature as the attic air).  But why does deep burying change the physics if air can go through the blow-in insulation on top of the ducts and contact the duct outer skin?  For that matter, the more I thought about this issue and confused myself, why doesn’t that roasting attic summer air just go right through the blown-in insulation and contact the back side of the ceiling drywall, creating the same potential for condensation?

I suspect the answer involves me better understanding heat transfer due to conduction versus due to convection.  But I suspect someone here can explain it.  Recommendations for an air barrier are always on all six sides, except for the attic ceiling.  What gives?  Thank you in advance.