Unventilated attics can help to mitigate energy loss through leaky ducts or ceiling fittings. The energy savings touted by unvented attics can be as high as 20%. However, if moisture is able to enter the unvented attic space, it may lead to roofing panel issues.
Placing notoriously leaky HVAC ductwork into an unconditioned space will not make for an energy efficient heating or cooling system.There are a few options which locate the ductwork within the thermal enclosure of the home. Options include moving the ceiling plane to the underside of the roof with an unvented attic or placing the ductwork in a conditioned basement. A third option is the installation of plenum trusses which allow for a compact ducting system to be placed in a conditioned bulkhead.
If your residential build occurs in building science climate zones one, two or even three, you may want to reconsider placing the ductwork for your air conditioning units in the attic, even if the attic is ventilated. Studies show you could be adding 15% or more to your energy bill with poor ductwork placement.