I'm installing properly sealed round metal ducts in my unconditioned attic for heating and cooling. I've probably got 100' total of 6" duct to cover. I'm trying to achieve an R-8 insulation value, but the big box stores only sell R-6 Frost King wraps. Can I use un-faced insulation batts (that you use in the attic), and then wrap a foil layer around it? Any other suggestions?