Thank you all to those who have commented on this thread. I am going to start others with some more specific questions as things move on, but this has been hugely helpful in making some key decisions about what to do now and what to leave until later. Mike, a huge thank you particularly for forwarding the links on IWI. The 'Historic Scotland' Case Study 4 was absolutely brilliant. What a great opportunity for fair comparisons between different technologies, and very reassuring to see that they all seemed to work in this set up without causing damp within the wall (which of course is the thing that most people doing this are afraid of). I would highly recommend anybody who is considering their options for insulating solid walls to read this also.

Posted By: djh

Posted By: NRDiggerThe 'Historic Scotland' Case Study 4 was absolutely brilliant.

Sorry, which was that? Do you have a link?

Case Study 1 seems a bit dubious to me. I've only skimmed it and not read it carefully but some of the numbers don't make much sense to me. Internal RHs in the 20% range? RH part way through the construction of 14.8% when all others are 60%+ except for a 16.4%?!!! Without some kind of explanation I would tend to discount all those results. And no discussion of how the various scenarios were air or vapour sealed (e.g. were plasterboard sheets tped and/or skimmed or not?).

I generally think Historic Scotland does good work, but my first impression of that particular study is pretty poor.

Ahhh...!! That was the study I was looking at (https://www.historicenvironment.scot/archives-and-research/publications/publication/?publicationId=651e6f16-087f-408b-ab3d-a59300fd971a)

In terms of the RH values, surely this is a matter of it being either right or wrong? I have to say I was surprised to see measures that low. Is the wood fibre board hygroscopic?

In any event, my TL;DR conclusion from all of this was to put Wood Fibre board somewhere near the top of the list as a reasonable solution for insulation of a solid wall from the inside. If that's madness, tell me now!