I’ve been wracking my brain trying to figure out the best possible option for my attic.  For reference, I’m in a climate zone 4A so attic needs to be vented to prevent ice damning.  I have one HVAC unit in the basement and one in the attic for the 2nd floor bedrooms.  My house is too old and not structurally strong enough to handle the weigh of a vented over roof as Joe L. details.  What are the failure points of a “vented under roof” if I were to install baffles from soffit to ridge, rockwool batts below the baffles, Intello plus smart vapor retarder and air barrier with taped seems below the insulation, and conditioning that room with a dedicated dehumidifier.

And yes, I’ve considered not having my mechanical system in the attic but in my small 1930s colonial home, there’s no room to run ducts from the basement to the 2nd floor, no room for a 2nd floor mechanical closet, and I want to avoid heat pumps or mini splits for EMF reasons.  My only other options would be window units on the 2nd floor.