Archive for the ‘house’ Category

Cavity Wall Insulation – Problems

Sunday, April 10th, 2011

My girlfriend is considering buying a 1930’s house.  It has evidence of dampness and cracks inside, against the outer and southern wall.  A surveyor mentioned it had cavity wall insulation but did not associate this with the dampness or cracks.  But I remember stories from decades ago where lacking-in-thought installations offering reduced heating bills etc. resulted in just these kinds of effect, by bridging of the wall-cavity.  Furthermore, in this part of the world, the southern direction is where the rain mostly blows from.  I’m no expert, but remember my parents’ repeated warnings about this issue – from their absorption of newspaper & TV coverage over several decades.  So time for a bit of Googling.  Best sites I found:


  • Potential Problems:
    • Cavity wall insulation risks internal dampness and accelerated corrosion of ferrous wall-ties (the latter e.g. by condensation).  Not a certainty, but certainly a risk,the “success rate” would be interesting, but for a specific case, it is more about proper pre-installation inspection for suitability.
    • Increased the risk of damp to the property could in turn cause internal decorative spoiling, plaster damage, fungal decay to  structural and joinery timbers and also accelerate the risk of corrosion to the metal wall ties.
    • Another problem is insulation sinking under its own weight. Over time, the fibre can compact and settle in the cavity, leaving cold areas at the top of the house, ripe for mould growth. Insiders say this is often due to insufficient fibre being used – a common problem with contractors employed on bulk contracts claiming the government subsidy.
    • Foam insulation can sometimes lead to cracking damage – usually to internal wall surfaces – following foam cavity wall insulation which might possibly be due to the expansive force of the foam as it cures.
  • The Failure Physics / Mechanism:
    • Internal wall dampness can arise where rain blown by wind hits external walls then the cavity insulation forms a bridge to the internal wall. This can happen even when the cavity material is itself water-resistant (presumably because of surface tension and capillary action etc.).
    • If the installation is less than perfect, leaving unfilled air pockets – these could leave ‘cold spots’ on the inside walls which attract condensation. Another problem concerns wall-tie corrosion; cavity insulation makes the outer brick leaf colder, and therefore wetter, which can accelerate rusting of the wall ties.
    • Aside from the damaging consequences of dampness, cavity wall insulation can sometimes actually lose building heat.  Blown mineral-wool fibre has to become only slightly damp (around one per cent by volume) to lose all of its insulation properties. Any damper than this, and it will actually start to draw more heat out of the house than if the insulation were absent.
  • Potential Misdiagnosis:
    • It feels like there is some blind-spot, if not denial, to this long-standing issue.
    • There is a tendency to assume damp-course issues, even when they are not relevant.
    • Another reported misdiagnosis is roof problems, the cost of (potentially pointless) fixing of which can be thousands of pounds.
  • Materials & Installation:
    • Various kinds of material can be used, the cheapest and most popular being blown “wool”, others being fibre, bonded polystyrene beads and foam.  Choice of material does not guarantee freedom from dampness risk.
    • In new buildings, usually the insulation is fixed only 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” (vertically-aligned grain).
    • Some properties are not suitable for cavity insulation.   Pre-installation inspection should include checks via a boroscope.  Even large cavities do not give freedom from risk.
    • Installations should be backed by a 25 year C.I.G.A. guarantee.  In any case, assuming that the company responsible for the original installation is still trading, then these costs may well be their responsibility.
  • Removal:
    • Removal under guarantee does not necessarily take place instantaneously…
    • Removal plus damage repair (internal plaster etc. and/or ties) can cost around one to four thousand pounds.  Removal difficulty/cost depends on the material, and can leave “permanent scars” on the building exterior.
    • All cavity insulation can be removed. Mineral fibres and polystyrene granules can be blown or vacuumed out, whilst solid foam insulation must be manually broken up and scraped out.
  • An alternative to “filler” types of cavity wall insulation: Celotex.  This is a type of insulation board that can be attached to inner or outer walls, external to the cavity.  Guides: