Hi all,
I am sure this is a silly/dumb question but I cannot figure it out. 

We are renovating a summer kitchen to be a conditioned office / ADU. The original 100+ year old building had 3×4 rafter falling on a single true 2×4 plate. 

My plan to insulate was to fur down the rafter with rips of of 2x6s we have leftover (see true 2×4 walls). What is really throwing me for a loop is that as I fur down the rafters with a few of these 1.75 inch depth boards they will quickly start falling below the top plate. 

Am I making this much too difficult? Do I simply cut them to have a plumb cut at the wall/studs and then sheetrock down to that?

Further does anyone have any thoughts on the best way to vent this whole assembly (It currently has/will have a metal roof of some sort over ancient sheathing)? I probably should replace the sheathing but haven’t gotten that far yet and its 12/12.

My plan is to use rockwool. Current plan was just to use owens corning vents in between each rafter but venting kind of intimidates me and I do not have a good handle on it. 

Thanks in advance!
Jay