John Rumm wrote:
Normally you would have ridge and soffit vents to provide airflow, then
battens fixed to the sides of the rafters to prevent the insulation
boards seating too far back in the gap. That way you get a complete
thermal envelope in the loft, but with the outside of the rafters, and
tile battens still ventilated.

Having done some more investigations, I've managed to burrow my way through
the insulation from the loft behind the wall. It turns out the 'cold'
bays are mostly uninsulated with some rockwool pushed right the way up
to the apex, while the 'warm' bays are full up with rockwool, with no air
gaps.

Some of them will be tricky due to conflicts with other things, but the plan
for the accessible ones is to hook out as much rockwool as possible and ram
up some 50-60mm celotex (120mm rafters). I might try and make some
celotex spacers to keep an air gap underneath the sarking.

I wonder what's the best way to stick small things to celotex, without
tearing the foam...

Theo