Hi All,
I want to remove the paltry fiberglass duct insulation on my HVAC duct work located in my unconditioned vented attic space (behind the kneewall).  It’s a terrible design, but reworking means a big hassle.  In addition to thinking about installing a vapor-variable air barrier on the underside of the rafters (2x10s with kraft-faced fiberglass batts filling the bays)–please advise if this is a terrible idea–I’d like to reinsulate the ductwork by sliding a layer of rigid foam under it to isolate it from the cold subfloor and then using a DIY wrap of lambswool batts and some type of facing.  Should the facing be foil or something more vapor permeable? 
NB: Batts removed for exploratory purposes in these photos, but we hope not to have to remove them throughout the entire attic space if possible.
Thank you for your help!