Vaulted ceiling that has half of the floor above outside and the other half is internal. How to insulate?

[buildlondon363]

On the right side ceiling above it is an external open porch area. On the left where rockwool is, is internal to the building and has a habitable space above. I am fixing PIR backed plasterboard to external walls with foam/ fixings going up gradient ceiling to lintles second photo. I am going to build a dropped ceiling with atleast 50mm void fill ceiling with 120 PIR then screw 25mm PIR onto the frame then board. I will vent this space through an air brick roughly above the bunch of cables then another far right which will shoot up the wall into some boxing in that is in the porch area which I will vent. This will provide some decent air floor but I don’t know if it will get problems ? I was going to PIR and and seal with some foam or tape around as the other rooms on this floor will be warm so no warm air gets in this void. I was also going to change that rockwool to PIR so it doesn’t rise up into that floor. Or I thought put a strip of pir where the timber separating the 2 sections are and have 2 seperate voids but where the cold one is just ventilate that? Then I won’t have any concerns about both crossing ? Seems a huge palava and weird scenario. This is a semi basement.
Thanks for reading hopefully hear some thoughts.