What you are describing is commonly called a hot roof systemand is actually my favorite type.
With that said, even if you filled that cavity full with closed cell foam like Demilic Soy-200 with an R Value of 7.4 per inch, you are right on the cusp to not meeting the minimum R-Value to prevent a common condensation issue. North Dakota falls in climate zones 6 and 7 meaning you need an R25 or R30 to help prevent that.
The next issue is that you don't have any insulation covering the 2x4's leaving open some thermal bridging issues which can lead to ice dams.
- If you can handle losing a couple of inches of headroom you can add in some foam panels below it which would help eliminate the thermal bridging and get closer to the R49 recommended / code in your area.
- If that is not doable you would have to remove the roofing and install some more insulation on top of the roof sheathing.
One final concern many have up in the northern areas (at least in the North East - Vermont, etc . . . ) has been issues with shingles cupping. Whether it is due to not enough R-Value or using open cell foam it is hard to say. If this is concern of yours you can always go with metal or consider venting above the sheathing or foam.
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