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 cav= ity, 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 satura= tion 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 a= t 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 regulati= on standards is considered to be defective, not compared to when it was bui= lt. So as usual a piece of toilet paper in most instances except where it c= an be demonstrated the installer was negligent in assessing suitability whe= reupon 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. Moder= n cavity wall insulation at build involves thick celotex (far better than g= lass wool) AND usually maintains an air gap so water penetration can run fr= eely down the outer leaf. A common fault then is the insulation separates a= way from the inner leaf, or a gap is left which results in convection - ess= entially turning the insulation's effectiveness into little more than "ware= housed 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 outpe= rform CWI in terms of rapid room warmup (and cooldown, more useful in summe= r) and give the lowest heating loss.