I'm still not sure but let's get our language the same. Ceiling or roof "joists" will span from wall to wall and will be flat like the base of a triangle. "Rafters" span from each wall to the peak of the ridge and form a point like the two upward legs of a triangle.
Typical residential roofs will have prebuilt trusses that have the ceiling joists tied into the rafters going to the peak with cross bracing. Then the ceiling joists have the drywall ceiling put on underneath them and then insulation is laid above the drywall in the ceiling joist area and the spaces between the rafters going up to the peak are left open.
Cathedral ceilings have no ceiling joists and the ceiling drywall is put directly on the rafters going up to the peak.
Since you say there is no attic above the 2x6's then I assume we are dealing with a cathedral ceiling. If that is the case, you absolutely need to install plastic air chutes to funnel air from the peak to the soffit vents. These take up some room so you can't get the normal r-19 5.5" batt in there.(r-19 5.5" insulation is designed to fit flush into a 2x6)
Now it depends on if you need a drywall ceiling put on or if you are just going to let the kraft-paper facing show. If you are adding drywall then you need shim the 2x6 with another 2x4,6,or8 depending upon how much insulation you want. If this is an open ceiling where you can see the insulation,(such as in a garage) then add a shim every few feet and use chicken wire to hold the insulation up.
Post back with you exact situation and if I have answered your question or not. If not, I'll try again. Good luck.