roof insulation thickness

I had a question about the thickness of the batt insulation in the roof. You asked us to make it thicker than our deepest joist, in my case 220. However, I am not sure what can hold it. For example, the image I attached shows that the joist is flush with the slant of the roof, if I give ore space for insulation what can I add so there wouldn’t be a gap above the joist? Double studs (47×75)? If so, how far should I space them from the joist?

See the attached sketch in particular the parts labeled A, B, C, and D.
Because roofs tend to be the surfaces you lose most of your conditioned air it is the area that should have the most insulation if possible. The overall insulation area should be equal or greater (B>A) to achieve this you can put your roof furring strips (D) at a distance from your rafter. To support your roof furring strips you can use 18mm OSB or plywood (C) to connect them to your rafters.

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