Mode of failure

I wrote to CIGA explaining that the 25 year guarantee certificate does not mention anything about the how having a cavity full of wool changes the mode of failure in terms of water ingress compared ot the original empty cavity.
It's a well accepted fact that cavity walls were designed to keep the water away from the inner leaf and problems arise when anything other than air sits in the cavity providing a bridge that could result in water ingress.
If you have cracks in the render then it may cause some water to enter to the inner surface of the external wall just as with wind driven rain. However the presence of insulation material can lead to bridging and penetration into the inner leaf, particularly if it there are gaps.  In the case of wool it can become saturated and never dry out leading to an obvious damp mass which readily transfers to the inner leaf.
I argue that with cavity full of wool, the mode of failure when presented with severe weather, or cracks in the mortar is completely different to an empty cavity. The later allowing the home owner time to schedule repair (if they can afford it at that time!) where as with a filed cavity -even a  single bad weather event could cause the wool to get wet and then destroy the internal plasterboard/decor leading to an expensive removal of insulation.

Customers should be clearly informed of this before agreeing to an install but also as the guarantee transfers to buyers down the road - the certificate should make this clear to future home owners.

CIGA's reply:

All cavity wall insulation products registered with CIGA are tested and approved by the BBA (British Board of Agrement) as suitable for installation to all regions of the UK to standard masonry construction types following a thorough pre-installation assessment. Whilst we acknowledge your comments regarding the theory of an empty cavity wall, CIGA has issued over six million guarantees of which 0.2% report concerns post installation, and upon investigation the issues are in the vast majority of cases found to be due to a lack of maintenance and consequential water ingress, as appears to be the case at your property.

CIGA have admitted that there were problems in the pre-installation assessment on our property.  To me they all point to the fact that a thorough pre-installation assessment could not have been carried out.  But also to call it a 'theory' of empty cavity wall is patronising.

Our independent damp surveyor who is Certificated Surveyor in Structural Waterproofing (CSSW), PCA Certificated Surveyor In Remedial Treatments had this to say about filling cavity walls and I think it speaks for itself:

What happens when you fill an empty cavity with insulating material? 

A bridge is formed between the inner and outer leaf which provides a passage for moisture to pass through. 

Cavity masonry walls were introduced on the exposed western coasts of Britain and Ireland in the nineteenth century, to stop wind-driven rain from penetrating to the inside surfaces. They gradually spread to other, dryer, parts of the country, because the air layer trapped in the cavity was found to provide a degree of thermal insulation. 

But the main reason for building cavity walls has always been to keep the rain out. 

Since the 1980s, the Building Regulations have required new houses to be built with insulation material in the cavity. As long as they are built properly, this insulation should not compromise the walls resistance to rain penetration.
In most cases, the insulation is fixed to the inner leaf, leaving a narrow cavity to intercept any rainwater that penetrates the outer brick leaf. This insulation material is usually in the form of rigid foam boards, which are intrinsically waterproof, or semi-rigid mineral-wool or glassfibre "batts", where the fibres are aligned vertically so that any penetrating rainwater should drain downwards in the cavity and not have the chance to penetrate across to the inner leaf. 

This is important, because the Building Research Establishment has found that single-leaf brick walls always leak when exposed to wind-driven rain. The leakage occurs at the vertical joints between adjacent bricks, because of drying shrinkage in the mortar.
It is not a question of poor workmanship; it is reasonably deemed an inevitable occurrence in single skin walls. 

People with single-leaf brick garages attached to their cavity-walled homes are known to complain that rainwater penetrates through from the outside when they are subjected to wind-driven rain, resulting in puddles on the garage floor therefore the same scenario will be occurring to the brick outer leaves of their living rooms and bedrooms, but as long as the cavities are left clear, the water should run down the inside of the brickwork to foundation level and never be noticed. 

Dampness problems caused by cavity wall insulation do not generally occur in houses where the insulation was built-in from new. They happen in houses which were built prior to the 1980s, with clear cavities, which have subsequently been filled.
The material which has attracted most complaints is reported to be blown mineral-wool fibre. This material consists of loose mineral- or glass-fibres which - as the name suggests - are blown in through holes drilled in the brick outer leaf. 

Despite the constant insistence by manufacturers and installers that cavity wall insulation cannot allow rainwater to cross the cavity, the Building Research Establishment has found that it can.
Their findings have been published in BRE Good Building Guide 44: part 2: "Insulating masonry cavity walls - principal risks and guidance" 

This states, "There can be an increased risk of rain penetration if a cavity is fully filled with insulation, ie moisture is able to transfer from the outer to the inner leaves resulting in areas of dampness on internal finishes.
Rainwater, under certain driving rain conditions, can penetrate the outer leaf of masonry leading to wetting of the cavity insulation and subsequently cause damage to internal finishes.