13 Feb 2012

Problems with cavity wall insulation

One of the first blog pieces I ever wrote, in November 2005, concerned cavity wall insulation (CWI), and examined the likelihood of there being problems. At that time, my blog got published in two different places which wasn't the cleverest idea I ever came up with as my own blog piece on this topic never got a single comment, whereas the other one gleaned no less than 85. If you Google cavity wall insulation problems it still comes up on the first page, despite being six years old.

This morning I sat down and read all the comments and tried to make sense of them. Most of them are observations and anecdotes, a few are pretty desperate cries for help and mixed in here and there are around a dozen cases of people getting mould or damp problems which they attribute to CWI. In every case, they have then received short shrift from both their installers and the guarantors. The issue seems to be that it's not possible to say definitively that their problems result from having CWI installed and so the buck is easily passed. But surely the chances are that they can't all be making it up, or jumping to the wrong conclusions. Something mouldy is going on here.

Now my article starts out with a dig at Jeff Howell who is the Telegraph's building agony uncle and who maintains a website with extensive articles containing his thoughts and observations. Had I known what comments would follow on from my blog, I wouldn't have been so critical of Jeff because it seems that he has genuinely tapped into a pool of discontent about CWI and, as he says, he is just about the only person giving it a voice.

Behind this are some big questions. What percentage of homes that have had CWI have gone on to report damp or mould problems? How can you tell which as yet untreated homes are likely to suffer problems? Have we adequately assessed the risks?

At the tail end of last year, DECC published the results of a study into CWI, looking at the issues with the many as-yet-unfilled cavities and how best to address them. It could have looked at issues with already-filled cavities but chose not to. Is this because this would have been off message — i.e CWI is a good thing, so why point out that it might not be? Or is the problem with damp and mould so minute that they feel it's not worth worrying about?

If it's the latter, then they could greatly strengthen their case if they were to publish the numbers of complaints received about problems resulting from CWI, rather than pretending that they are irrelevant. The internet (and Jeff Howell's postbox) is full of dark rumours, and it's high time we shed some light.


  1. Ted MaryniczFebruary 13, 2012

    Some TV consumer programmes have followed this. BBC Wales X-Ray got refunds for some owners last year IIRC.


  2. Poor idea anyway - CWI is a stopgap half measure (if that) relative to the standard of insulation requirement that is rapidly taking root - i.e. a minimum of 200, preferably 300 of EPS (expanded polystyrene) or equivalent EWI (external wall insulation. CWI has only had a look-in because dirt cheap and do-able now.

    In future it will become widely understood that thermal massiveness is just as important in effect as is thermal insulation; if used with understanding, lots of massive material in contact with the interior environment has great benefit, and this will become part of the standard toolkit in treating the existing building stock.

    Unfortunately CWI has the effect of robbing building interiors of half the wall-massiveness that they have, by isolating the outer skin from the interior environment. So in future it will not just be a matter of topping-up CWI installations with additional EWI, but of removing the old CWI, prob replacing with weak concrete or something massive and conductive, to create 250-300mm of solid thermal massiveness available to the interior environment.

    Still-empty cavities will be an asset in the future!

  3. @fostertom
    Thermal mass does play a role in moderating temperature fluctuations, particularly in relation to summer overheating, but its role and importance is not at all comparable with thermal insulation.

    1. I agree with Max and would like to add that thermal mass generally needs to be exposed, so the outer leaf of your cavity is really not useful thermal mass. I had an argument with an architect once who thought the outer leaf of his *insulated* cavity wall was useful thermal mass.

      If you're planning on spreading heating/cooling loads over the 24h cycle (to take benefit from night time cooling etc) you only really benefit from the first 75mm of passive thermal mass (and only if there aren't finishes in the way).

      So the question is, who likes exposed concrete?

      P.S As a structural engineer I do not suggest anyone tries to fill their cavity with concrete unless they have taken professional advice - it may not be a good idea.

  4. Way to go yet, in exploiting thermal mass - for serious solar storage for example, in ways that get well round the present typical shortcomings of storage as presently understood and practised.

    E.g. heavy walls inside EWI jacket, solar heat fed into outside face of the walls, takes a fortnight to trickle thro, nicely damping out the big fluctuations inherent in winter solar collection/input.

    That's a v different use of mass for storage, than the typical one where sun comes in thro windows and falls on heavy walls/floor - waste of time making the mass more than 100thick in that scenario, I agree.

    1. Oh, you just answered my comment before I wrote it!

      I see what you mean, but I still think I'm with Max on insulation being a higher priority.

      As for adding thermal mass, unless the existing building has very strong foundations the best way would probably be with a phase change wax or similar (which has a lot less real mass to trouble the foundations). I remember seeing an article about plasterboard with wax pockets to absorb & emit heat at their melting/hardening point.

  5. Is that warning serious Fred? I'd have thought that most masonry buildings up to 2 storey maybe 3 wd be well oversized foundation-wise, and that dominated by live rather than dead loads? So you'd have to add a huge amount of mass before running out of bearing capacity?

    Obviously, trying to make a lightweight stud-frame or clad frame building massive, or trying to make joisted floors massive - that's something else.

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  8. gap in cavity is only 40ml. nobody seems to be able to work to this tolerance

  9. check out BASF foam fill system. 40 mm shouldnt be a problem according to the website. should rectify any problems with rusty old wall ties too - sweet. Every building should be assessd for suitability regarding cwi, but I still dont see how evenly filled cavities can cause damp if the infill is water repellant. Once moisture held in the masonry on inner leaf has got free - probably thru a damp period inside the dwelling - sureley the masonry and plaster would settle. I suppose big bubbles in the cwi may lead to condensation within the cavity, but moisture there must be able to get out the way it got in?! seems to be most folks have lofts done at same time as cwi, then talk about damp on upstairs ceilings near external walls but blame cwi not loft lagging. if cold loft is damp it isnt the cwi!!

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  11. If you have any borderline dampness issues it would make sense to address these first before insulating (90% of the time it seems to need better ventilation).

    If you have an older house with a higher risk cavity it is not hard to take a brick out of the outer leaf and check the condition of the cavity yourself - it should be clean and the bottom should be dry.

    Fully agree that its worth paying a few hundred extra for (platinum grade) polybead. This flows beautifully.

  12. When I look at cavity wall problems and compare with the supposed 'pros', I don't think it's worth having to be honest.

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