Great to hear from you Mark and I am glad you think filling the cavity could be the wise thing to do….. or another option could be sealing off air vents? Though the cavity may still be connected to sub floor or roof spaces and the overall result wouldn’t of course achieve such a good U value.
Surely hygroscopic infill insulation material would be OK if the external weather screen on EWI is performing adequately? I am keen on using a detail recommended by Natural Building Technologies http://www.natural-building.co.uk/sites/natural-building.co.uk/files/pdf/8/retro-ewi-clad-system-sheet-may-16_0.pdf
Link to the paper I mentioned: