Insulation detail over gable wall

sas

Building passive spec house, Block on flat with EWI

Roof build up will be as follows (from outside):
Slate
Slate Batten
Counter Batten
Breathable membrane (I'm aware this is not necessary but I'm a belts and braces kinda guy)
100mm Softboard
225mm rafter fully filled with cellulose
Airtight membrane
Service cavity (may be insulated)

My gable walls finish just below the base of the 225mm rafter, i.e. my rafters pass over the gable blockwork. The issue I am looking for thoughts on is what insulation to use in the space directly above the gable block work between the rafters.

Cellulose can't be practically installed there because the space between the rafters won't be accessible at the time the cellulose would be installed.

So I'm working on the assumption that I'll need to insulate it before the softboard goes on. This introduces the added element of rain. If as the softboard is being fitted it lashes down I'm assuming rockwool etc. would be destroyed. So would you recommend that I use a rigid board just at this point? The added bonus of the rigid board is the better u-value at that position. In the event that I need to insulated the service cavity, the service cavity insulation will not pass over the gable blockwork. So 225mm of PIR etc. would counter act this.

I realise that I could simply go with a wool type insulation and wait for a period of no rain. This is Ireland and we are in september however...

All thoughts appreciated.

sinnerboy

Use rockwool ... and a tarp over

soldsold

I used PIR boards in my case, one gable wall was being externally insulated.