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I dream of a cavity wall tbh! It would make our house a lot warmer and easier to insulate.
Is this purely because you can get cavity wall insulation? (With a cavity)
andos-
No, I'm no expert on cavity walls but I think they were mainly introduced to reduce damp problems. But the other advantages of having the air gap between two walls are inherently better thermal properties (even without insulation), better soundproofing and a handy pre-made space to fill with insulation.
Fox
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 😵💫