Insulating in eaves/void behind stud on '90's build

[mm289]

Evening all, having finished the C18 farmhouse a few years back now starting to upgrade a 90's build terrace which is colder than my farmhouse was 🫣

 

The place has had cavity wall insulation fitted historically and has about 200mm Rockwool in the roof but apart from that 🥶

 

We are starting to expose the top (3rd) floor as we replace bathrooms and as suspected have found that we basically have stud (with some 50mm polystyrene) then a rather massive air gap (1500mm) to the felt/roof.

 

The pic below shows the stud wall behind the bath which is all going to get pulled out to create access

 

Between the 2nd floor ceiling joists is some loose fitting rockwool and that is about it  - above this room is the loft. This pic shows the eaves void, with the rafters running up to the loft/ridge beam and the 2nd floor ceiling joists below.

 

 

So my plan is to get into the eaves and start by:

1. laying 200mm rockwool between joists
2. Foam up where joist ends go into wall/blockwork

 

I initially thought I would just be able to fit Rockwool bats between the studwork when we replace it, but having taken the plasterboard off the gap in the eaves is huge, so am now wondering about putting PIR in between the rafters as well?

 

Problem is, because of access issues I will never be able to make this area a proper "warm roof" so is it worth trying to insulate the roof structure or focus efforts on insulating the stud?

 

Any thoughts gratefully appreciated (I think it was easier working with 18" solid brick lol)

 

Cheers,

 

MM