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.

  • You seem very concerned about the CWI acting as a moisture barrier inside the wall itself, preventing moisture migrating through the brick either inside or out. I'm not familiar with CWI, but your description of "blown fibre" indicates to me that it's probably some sort of cellulose or fiberglass based insulation and, therefore, has effectively zero propensity for blocking moisture transfer. If I were you, that's the angle I'd investigate further. Even if it does block moisture, is it a full, complete moisture barrier between inside & out? If not, moisture will get through.
    – FreeMan
    Aug 10, 2023 at 12:10

1 Answer 1


I've had a verbal conversation with someone I'd regard as an expert in this field (they happen to be externally insulating their CWI walls). I'll try to summarise:

  • We have "thin" cavity walls, in so much as the cavity is relatively narrow, and so the CWI in it is not as effective as if it were thicker. There is room for improvement ;-)
  • Exterior insulation would likely be expanded polystyrene, (screwed/glued to the bricks) which can be rendered directly (there are "brick cladding" systems available too, which may nor may not be useful if aesthetics don't allow for rendering). Beware windowsills would need extending.
  • Interior insulation can either be insulated plasterboard (which has it's own vapour barrier), or can be foil-backed solid insulation with regular plasterboard over the top (likely with battens between the insulation to screw the plasterboard up), all taped up to form a contiguous vapour barrier. Beware plugs and pipes will all need moving.

One phrase I heard was "you've got a cavity wall, you can do anything you like - I wouldn't be saying this for a solid wall". I also heard it's possible to "opportunistically" do (say) the small end of a rectangular room, and then come back years later and do the long side of the room (assuming you can accept the work taking place).

Whilst this gives some comfort that CWI walls can be improved, and gives a path to doing so, it's not clear what happens if there is moisture in the brick work, perhaps before the works, or occurs because of a leak or whatever. I think all moisture can be expelled outwards, but given the CWI in the middle, that seems unlikely for the interior layer of bricks. Whatever the mechanism, it seems CWI allows for a few different options for improvement.

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