Advice needed on loft insulation and boarding please!

Wrong forum, you're in the projects forum which isn't where questions are supposed to be asked. I'd ask a Mod to move you to Building or roofing.

The foam insulation isn't going to do much, it looks thin. What it may do is block any airflow through your loft space. That isn't normal for a pitched roof. Normally they're built as 'cold' roofs. Cold roofs allow airflow through the loft area to avoid damp build-up, just like air bricks do under the house.

The alternative is a 'warm' roof, which doesn't have the ventilation, but requires a different design that I'm not capable of commenting on.

Are there any holes at the edge of your roof that would allow air through or is it sealed tight by the foam?
 
I've no idea on this wind load stuff, it's beyond my skillset. Have you checked that your roof is tough enough for the extra weight of the chipboard?

For the insulation you should be looking at around 300mm of rock wool or around half that of PIR. Since you have a draft (good!) then the normal approach would work fine.

If you're getting boards put down I'd be looking at filling the joists with rockwool, covering with your reinforcement chipboard and then overboarding that with 50mm or 100mm of PIR to bring you up to the right standard.

At the edges go close to the edge but leave 50mm clear space to keep those drafts blowing.
 
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