CWI, Impermeable masonry paint, hard cement and breathability

Waterproof or let's call it plastic masonry paint made with silicone or acrylic sounds like a fantastic idea to keep that pesky rain away from the wall. Unfortunately it can easily cause the opposite in terms of moisture management and inflict devastating damage to old solid walls which were originally made with a breathable building envelope.
This article serves as a discussion to highlight some of the unexpected problems which show up in the real world verses the limited testing of the BBA certificates and I welcome discussion from anyone interested in this subject.

For a good 'primer' on the problems associated with plastic paint in solid walls, Peter Ward from Heritage House has a number of videos of detailing the sacrilege caused by often well meaning tradesmen applying these aggressively marketed products. I recommend watching all his videos especially if you are having any sort of damp problems so that you can avoid the multimillion pound interventions sold by 'damp proofing' industry.

Our house is 1930s and is composed of two parts - the original 1930s cavity wall structure made with soft field bricks and lime mortar topped with some sort of lime rough cast, and the second part is 1960s extension made with modern cementitious materials.
Both had been covered in what looks like an acrylic impermeable paint before the cavity wall insulation was put in. This can be shown because some of the difficult to access CWI plugs are unpainted.
One of the problems we have  is that the paint has started to flake and there are areas around the cavity wall holes which are letting water under the paint and in some places into the masonry.
When inspecting the inside of the cavity I've even noticed mortar plugs that have been pushed too far and  poking well into the cavity causing potential bridging.

When I removed the paint around the holes it showed cracking of the render around and some trapped moisture behind the paint which appears to have entered via the edges of the drill holes.
This is worrying because trapped moisture will be subject to freeze/thaw cycles which can crack not only the paint but the render substrate and soft brick. This will be exacerbated by the insulation because CWI by design will result in a much colder external wall which is more susceptible to frost damage.

Poor match of mortar for CWI holes causing point of moisture ingress into the masonary.

Poorly matched plug hole causing cracking of the render.

Portland cement plugs play much better with modern render 

The installers clearly used the same mortar to plug all the holes in all parts of the the house but hard portland cement is incompatible with the lime heavy render.

Impermeable paints and Cavity wall insulation

Peter has shown that waterproof paints are terrible for old solid wall buildings but my thoughts are that old (unfilled) cavity walls might tolerate them better. The reason being  that a well ventilated cavity might help evaporate moisture that would otherwise remain trapped. This facility is, however,  taken away when the cavity is filled and the cavity vents sealed up.

I have found the following important sources which state that water impermeable surfaces may be unsuitable for cavity wall insulation:

“Walls painted externally with paints that are impermeable to water vapour may mean that they are unsuitable for cavity fill, in these cases walls should be carefully assessed by the installer prior to any works carried out. “ - Government’s Housing executive Cavity Wall inspection report 2014 

The Energy Savings Trust also state this advice word for word:

"Walls painted externally with paints that are impermeable to water vapour may mean that they are unsuitable for cavity fill, in these cases walls should be carefully assessed by the installer prior to any works carried out"  - Saving Energy Trust CE252 Cavity wall insulation in existing dwellings: A guide for specifiers and advisors

In an different version of this document from 2002, the Energy Savings trust are definitive:

"Walls painted externally with paints that are impermeable to water vapour are unsuitable for cavity fill . Energy Efficiency Best Practice in Housing

What happens when you block off the cavity vents and fill it with the dreaded wool?

The sources above do not go into any details about the problems but it's easy to see why there would be concerns.

I'm not a buildings expert but It seems reasonable to suggest that without ventilation in the cavity,  moisture from inside the building will travel through the vapour permeable insulation and settle on the cold external wall (or within the insulation if that's where the dew point is), Here it can't escape through the outer skin due to the plastic paint. This will also be true of any penetrating rain that the empty cavity is meant to protect from.

Prolonged wetness in the external wall presumably weakens the mortar and may cause deterioration through freeze/thaw cycles. It may also increase the moisture content within the mineral wool insulation which at some point stops insulating and starts causing a thermal bridge.

ENERGY EFFICIENCY IN HISTORIC BUILDINGS Early cavity walls - English Heritage Report 

The CWI drill holes in the old part of the house are dreadful and It's highly likely that CWI put into a building with impermeable paint significantly accelerated the render damage. This would certainly explain why this very old render degraded so quickly in the last 9 years since CWI was put in.

CIGA Guidance

I've not found anything specific relating to impermeable paints in CIGA's 2002  Installer documents. But the following section details the procedure for rendered surfaces.

Technician’s guide to best practice Version 2.0 - Issued July 2002

On the CIGA Web site you can also find  the document "CIGA & PCA Cavity Wall Insulation Consumer Information" which outlines a 'maintenance schedule' for a property. [The document conveniently omits published date on the document but the Adobe publishing info says it was created in Aug 2017. I.e a bit late for the majority of consumers with CWI who are retrospectively being told how to maintain their property correctly]

During periods of heavy rain, even seemingly minor problems with rendering paint and roughcast can allow significant amounts of water to enter the walls. This can be made worse if otherwise waterproof surface coatings prevent the walls from drying during dry periods - CIGA/PCA

What do CIGA say?

I'm still waiting to hear back from CIGA on the cracking and ingress at the drill and plug sites, their last correspondence has against stated 'maintenance'.   This paucity of words puts me in mind of Hodor the mono-word simpleton from Game of Thrones.

The industry knows about the problems mixing CWI and impermeable paint. CIGA have made a nod to it in their 2017 guide and evidently someone in the Energy Saving Trust though it important enough to include in their document. In my experience the industry waves the BBA approved status of a product when it suits them as if it is self evident proof that the product was suitable for all building types. As a public we have very little information on how the simple tests in the BBA certificate were justification enough to puncture and fill 6 million or more varied and idiosyncratic buildings.

It's also important to know that you can have BBA approved CWI and BBA approved 'waterproof' paint on your exterior but it doesn't mean that the whole system has ever been tested together in the real world.

The public might initially think that the idea of getting building control approval for a simple task such as painting a house is overblown and unnecessary, however as shown a poor choice of materials influenced by aggressive 'Solution' marketing and lack of understanding in both consumers and trade people can lead to real structural damage to older properties.
Both CIGA and local building control advised me to put water proof paint on the house (though the later admitted to not having any experience with CWI). But this advice runs contrary to the sources above and so I've contacted the Energy Savings Trust to understand a bit more about why they included this specific guidance in their report. After all Saving Energy Trust logo appears on my CIGA guarantee.

This is a great example where home owners who try their best to maintain their houses may inadvertently make things worse by 'protecting their house'.  This fact was never explained to them when the CWI was put in and given that people in the trade who should know better don't understand it: what hope does the average consumer have.