CWI inspectors tend to miss key problems...
- Dirty narrow cavity, eg, 15-25 to 35-40mm rather than clear 50mm width
- Corroded wall ties, eg, black mortar combined with wind driven rain
- Brick returns, eg, at random to brace not just openings but outside walls
Corroded wall ties, narrow cavity, brick snots & brick returns bridging cavity, black hygroscopic mortar are all problems for CWI.
The idea is moisture may get in, but as it got in - so it can get out. This is not true in certain cases, some houses aged roof felt has flopped back against the eaves and dump water down the cavity in addition to normal wind driven moisture ingress. The problem is water loading builds up and saturation can occur.
I recall 2% moisture content of glass wool insulation results in 100% loss of insulation value. Where cold bridging occurs local condensation occurs at those points (typically around windows) which can cause decoration damage.
The CIGA guarantee is worthless because it does NOT cover damage caused by Building Defects and any building which is not to current building regulation standards is considered to be defective, not compared to when it was built. So as usual a piece of toilet paper in most instances except where it can be demonstrated the installer was negligent in assessing suitability whereupon I think they do pay for it to be scraped out of the cavity.
Bonded polystyrene ball insulation may outperform glass wool, never checked.
If cavities are 35mm the benefits are still present, but quite small. Modern cavity wall insulation at build involves thick celotex (far better than glass wool) AND usually maintains an air gap so water penetration can run freely down the outer leaf. A common fault then is the insulation separates away from the inner leaf, or a gap is left which results in convection - essentially turning the insulation's effectiveness into little more than "warehoused insulation board".
If you are redecorating a room, and you can afford to lose 50-70mm off it, frankly I would insulate internally with 50-75mm Celotex. It will far outperform CWI in terms of rapid room warmup (and cooldown, more useful in summer) and give the lowest heating loss.