What exactly is the problem with Cavity Wall Insulation?

Failure Modes in Blown Fibre Insulation

I suspect that the day of reckoning is fast approaching for irresponsible Cavity Wall Insulation installers but to date the campaign and public outcry against CWI failures has primarily centered on poor specification and installation. However, I think there is another very important question that should be asked… for those properties that were subject to a reasonable level of due diligence, responsible pre-survey work and sound installation methods, is it possible that even those installations may have failed despite all the installers best efforts? Sadly, the answer is a resounding yes and it’s an answer I give based on pragmatic experience on what I’ve been finding on site for a number of years.

I have been warning my clients against using blown fibre cavity wall insulation for quite some time because as part of the damp investigation process I frequently open up cavities or at least inspect them with a borescope. I firmly believe that blown fibre cavity wall fill is responsible for the vast majority of failures relating to CWI installation and I generally discuss two modes of failure in my reports.

  1. Dry Slump: Dry slump occurs within a very short period of having the blown fibre installed and how much it slumps depends on how well it is installed. A loose fill will slump significantly more than a tightly packed fill and you would be amazed at just how quickly this slumping can occur. I have rechecked properties within a week of first inspecting them with a thermal imaging camera and found that the walls now contain zero fill within the top 12 inches of the wall. Where a previous problem existed with cold surface condensation, then homeowners find that the problem revisits them but with a focus on the top sections of the internal walls that no longer benefit from CWI due to its slumping effect.

Realistically, all properties would be best having a follow up visit to top up the fibre CWI after it has slumped but this rarely, if ever occurs.

  1. Wet Slump: To my mind, this is the biggest problem with blown fibre CWI because when this mode of failure occurs it creates a problem above and beyond the obvious lack of thermal improvement to the building fabric. Blown fibre CWI, unlike blown polystyrene beads, is not inherently waterproof and I consistently find it completely slumped to the bottom of the cavity in a wet mushy mess. If you imagine holding up a bed quilt filled with blown fibre, holding it by two corners. You’ll probably find that the fibre is evenly distributed throughout the quilt. Now if you were to get an assistant to saturate the quilt with a hose-pipe you would quickly find that the saturated blown fibre slumps to the bottom of the duvet case in a wet soggy mess. This is essentially exactly what happens within the cavity wall, particularly for those properties that have an issue with penetrating damp to the outer leaf of brickwork. The resultant technical problem is that you will potentially have a very serious wall base damp problem caused by bridging of the cavity wall and physical damp proof course and of course no benefit whatsoever from the alleged improvement in insulation.
Screen Shot 2015-07-19 at 3.36.28 PM

Dry and Wet Slump to Blown Fibre CWI

Aside from wet or dry slump, I consistently see the problem of inconsistent and patchy fill. This problem is fairly obvious under thermal imaging and it is more unusual to find solid and consistent levels of fill than it is to find empty voids in the cavity. I think this problem has been fairly well reported and I’ve seen a number of thermal images online aside from my own that highlight the problem.

I notice that the ‘all damp is caused by cavity wall insulation’ party are using this piece to promote their cause and whilst I have no objection to that it should be read in conjunction with this piece to bring a sense of balance to the discussion… Is all cavity wall insulation bad?


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  • John-this reflects my experience with retro cwi(especially the fragmented fibre-Knauf and Rockwool). If you have a look on my website under “media consultant” there is some footage of this wet pulp coming out of a house in Hastings that I filmed for the BBC “One ” show a few years ago. The fact is ,fragmented fibre gets wet, and transfers dampness across the cavity. It also holds it like a sponge! Whole scale extraction is now taking place across the Uk,often by tradesmen that were previously installers ,and have seen the opportunity to achieve more profitable returns by undertaking extraction, rather than filling(where circa £100 a house was the norm!).

  • Phil Beynon

    Based on the article and my wider experience, it appears that the base material is “not fit for purpose”, if settlement of whatever kind occurs within what may be considered a reasonable lifetime, be it 25 years, 42 years or the lifetime of the property.

  • Joe Malone

    I would completely agree that blown fibre is not fit for purpose Phil, it is not inherently waterproof and was clearly chosen for purpose under the illusion that cavities remain dry and of course they don’t. John, you have have quite rightly pointed out the great irony, those that caused the problem now stand to profit from undoing work they did that was not fit for purpose and has even caused consequential damage to thousands of homes.

  • Phil Beynon

    Interestingly, the industry accepted statistics for failures has recently gone up by 100% and that in itself is in my opinion a massive understatement.

    There is also a significant and notable shift concerning the quoted lifetime of the installations, with a move away from “… It will last the lifetime of the property…”

    I suppose that there are a few aspects moving forward that will leave a bad taste for consumers; the first is that the failures are occurring well before anyone anticipated; the same businesses that put it in are taking it out; and of course, those that guaranteed the first installation want paying to guarantee the future installations.

    All good reasons for supporting the founders of CIVALLI, as yet gain they are taking the industry on with the help of DECC.

    • Joe Malone

      I’m not quite sure what your opinion is based on Phil but our opinion is based on facts underpinned by a business that has carried out hundreds of detailed damp investigations including expert witness work and our opinion is the CWI failures are drastically overstated. We had hoped that CIVALLI would bring a sense of balance to the discussion but sadly they don’t; they are not interested in facts or evidence, just sensationalist news stories that support their position and overstate the case. They may well have good intentions at the core of what they do but they are misguided and isn’t it interesting that they had to call us in the hope that we could provide a ‘victim’ of CWI failure in the EAST Midlands to support another sensationalist new story because they weren’t aware of one. Very odd that with a problem that is allegedly ‘massively understated’ that CIVALLI were not aware of a single failure in the whole of the Midlands.

  • Phil Beynon

    Hi Joe

    Your reply appears to have taken some consideration, with a 9 month delay; but better late than never.

    Your current position seems to differ from your position of 1 August 2015 when you stated in this discussion that :

    “I would completely agree that blown fibre is not fit for purpose Phil, it is not inherently waterproof and was clearly chosen for purpose under the illusion that cavities remain dry and of course they don’t.”

    Bearing in mind that around 80% of CWI installs were fibre there seems to be a flaw in one or other of your arguments as they appear to be at opposite ends of any reasoned assessment scale.

    What has technically changed for you?

    I am guessing that it could not be the 2016 BBA-NHBC abject test failure of fibre, just because they installed a defect free window in the test rig particularly as all the other systems tested passed the same test; surely it cannot be that BBA and a hand picked installation team could not under laboratory conditions install a few m2 of fibre to the correct density.

    Just in case you missed my many postings; I have always said that a correctly assessed and correctly installed CWI system that has passed all relevant and appropriate tests should perform as designed delivering benefits to the household.

    it is a shame that CIVALLI did not contact me as I could provide many examples of CWI failures in East Midlands.

    • Joe Malone

      No, Phil, my response took very little time and would you believe that I don’t permanently man this blog. However, I recently noticed your comment with regard to supporting CIVALLI and I was initially optimistic and supportive of CIVALLI, hoping they would move the issue forward in a balanced and reasonable manner but I realised that this is not the case and so personally I do not support CIVALLI.

      My position with regard to the technical argument has in no way changed. Blown fibre is not inherently waterproof and therefore is not fit for purpose. We should not be using it now but does that mean we should embark on a Nationwide campaign to remove it all? Absolutely not! Pragmatically, whether full fill is being achieved or not, it is still doing something to raise internal wall temperatures and conserve heat. Sure, there may be cold spots, but without the CWI the whole wall would be cold. I make these points very clear in my blogs. There is a potential for failure with wet slump and penetrating damp in some properties but this is relatively rare and for those properties it should be removed, but only after evidence proves the case. There is no doubt in my mind that for many properties, where CWI is removed they will now see an increase in cold surface condensation and realise that their CWI wasn’t the problem after all.

      There is an industry being created to remove CWI and our experience tells us that the problem is being significantly overstated. Bear in mind that we do more detailed damp investigations than most consultants in the UK. If your position is that all CWI should be removed then it isn’t a very realistic one. It’s interesting that you claim to know of “many” CWI failures in the East Midlands and perhaps you can provide us with the evidence of these failures? CIVALLI made such claims to me yet when I asked to look at that evidence they claimed that none of this ‘proof’ could be shared. This begs the question… Wouldn’t a consumer group who claim to champion the interests of the down trodden want to share as much information as possible to promote and support their cause? Apparently not! Then there was the claim that no consumer can afford a professional survey to prove this issue. So we offered our services free of charge to a needy case and would you believe, there was no uptake. Could it be that there was a concern that the evidence wouldn’t support their quite outrageous and over inflated claims? If you have evidence of ‘many’ examples in the East Midlands then I would ask how many? Also, for each case how have you proven that CWI is the cause of the problem? Who paid for the survey work that CIVALLI state no one can afford?
      As I’ve said before, we hold the middle ground on this issue and we certainly wouldn’t contemplate advising our clients to pay thousands of pounds to have CWI removed just because there are voids or dry slump in the system. Pragmatically, it is simply better to have it topped up. We also advise our clients to never install blown fibre where it is not already installed but I still think it is irresponsible to start a scare mongering campaign devoted to a National CWI removal programme when it still benefits the majority of households.

      On a final note… I’m unsure why you claim to not be aware of my technical position since I make it very clear in this blog. http://buildingdefectanalysis.co.uk/damp/should-we-demonise-all-cwi/ A blog I have to assume you must have read since you referenced providing CIVALLI with examples of failures in the East Midlands.

      • Joe Malone

        In response to my last reply to Phil Beynon, I received a long reply from Claire Eades of CIVALLI, which I have refused to publish on the basis that this is a technical blog and not a political arena.

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