how to insulate attic that contains high-efficiency furnace?
How big is the attic?  One solution is to take 2, 3 or 4" insulation board (R13 to R25) and affix it to the rafters.  This would basically make your attic conditioned space (and thus the air between the foamboard and your ceiling will also add R-value) while also protecting your ventilation pathways.  A board with a radiant barrier on each side would be a plus and be sure to seal between the boards.  If you Google "Superattic", you'll find a better explanation.  NOTE:  On a DIY basis, it's not very expensive.  But if you hire someone and your attic is big, the cost can add up.