Summary: Can I internally insulate a CWI-filled cavity wall? If I can, and I've done that, can I externally insulate it as well?
I have a 1920s-1930s brick built house, with render on the 1st floor (just regular brick on the ground floor). We have retro-fitted blown fibre Cavity Wall Insulation (CWI) throughout. Let's assume the CWI is well applied, or if not that it's been fixed as necessary.
One room we've yet to refurbish has some original(?) internal insulation. This all needs to come out as it's a fire risk by modern standards, but I'm wondering about putting some modern internal insulation in its place (it seems relatively "easy" as there's no plaster on the internal bricks, all the pipes and cables all have to move anyway).
In principle this seems like a good idea ("all insulation is good", right?). However, if we insulate internally, there'll be little to no heat from the interior to permeate the brickwork to expel moisture from it. I also assume the internal insulation forms a barrier to prevent moisture travelling from outside to be dried out inside. Are we likely to get damp issues in the brickwork if we do this internal insulation?
The second part of the question (rather more far-future, project-wise) is what about external insulation? Some parts of our house have been rendered, which (in theory) we could remove, insulate and then re-apply (lets just "hand wave" away any issues about the aesthetics or size changes for now). Could we additionally do this, even outside rooms with internal insulation? Would this then create even less ways for the brickwork to expel any moisture? Surely any heat from the Sun would be largely reflected, and there'd be little to no heat coming from inside either.
I've read a lot on this subject, but can't find any decent information about improving CWI-filled cavity walls. As best as I can tell, once you've got CWI no one seems to suggest doing anything else, even though modern building standards have lower U-values to a CWI wall.