Recognised Construction Detail for Gable

[Jammy5]

My SAP calc assumes I follow the recognised construction details. On the gable end of the property, I am expected to installation insulation above the fire socks and flowing it into the soffits and fascia. The sketch is below showing this:

 

https://www.recognisedconstructiondetails.co.uk/library/document/mpf-150-e13-02-gable-insulation-at-rafter-level.pdf

 

The question here is, do I really have to install the insulation in this area? 

 

Looking at the guidance photos, the SAP assessor does not receive a photo of anything other than inside the wall looking at the insulation between the truss/joist and internal leaf of the gable wall.

 

I was advised by someone else they didn't think it was best practice to fill the soffit/fascia with rock wool?

 

Any thoughts please?