A radiant barrier theoretically reflects 96% of the Infra-Red energy that hits it and has no effect on heat transfer by conduction or convection so it would not be cost effective in your situation. It works most cost effectively in very hot climates to reduce heat gain from roofs in the cooling season.

Add as much insulation with as high an R value as you can afford and will fit. You might try insulating the rafter bays. Foil-backed fiberglass would give you a radiant-barrier at that location at a low cost.

Insulation can be added to the top of the roof; on flat roofs, it can then be spray coated with a finishing material.

Thanks very much macy and worthy. Would adding additional insulation to the rafters really help THAT much vs trying to maximize what I can put into the attic floor? I am sure the attic does have some air transfer/leak in it.

Have a blower door test done on your home to determine
leakage sites. Seal all leakage between attic and conditioned space, then insulate.
radiant barriers work well in hot climates but savings
in heating climates are minimal.
adding insulation to the roofline (between rafters)
is usually done by spray foam in the attic.
cost is a factor, as is total sealing of attic.
also you would need to determine what type of
foam you should use. Open cell has less R-value per inch
than closed cell, but open cell will allow water to pass through and closed cell will not.
www.buildingscience.com has climate specific information
check out what they say about unvented attics and insulation for your region.
Best of luck.