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will you be rendering the external to prevent moisture from coming inwards? how did you calculate dew point or did builder?
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Have you got a plan to deal with the moisture from that internal air flowing through timbers or breaks around partition walls, sockets, joints etc then condensing when it meets your wall which will be colder due to the IWI?
My worry with this setup would be that the PIR will simultaneously stop your walls from drying towards the interior and lower heat flow through them, reducing their ability to dry and upping their moisture content. Which will speed up the rotting process of any timber in the wall.
This coupled with the point @apc raises about the lack of an external finish to keep rain out of the wall does seem to present some risk factors for interstitial condensation to me based on what I know so far.
Not wanting to poo on your parade at all and hopefully this is helpful*. You've clearly done a lot of research and IIRC you're getting a MVHR system which might make this theoretical internal moisture exactly that?!
*In due course I'd quite like some constructive criticism on whatever system I decide on because this stuff is flippin' complicated 😵💫
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apc
Fox
@ectoplasmosis
I'm about to put in interior insulation on a few external walls in our solid-walled Victorian semi, and will likely use PIR instead of wood fibre.
Going by various calculators, the dew point will remain inside the PIR sheet itself, and applying the sheets properly, using foam all round, finishing with airtight tape, will/should prevent moist internal air from ever reaching the brick behind the boards. 50mm PIR will/should perform much better than 60mm wood fibre, and is significantly cheaper to boot.