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Damp problems associated with cavity wall insulation

Based on an article by Jeff Howell of the Sunday Telegraph.

Cavity masonry walls were introduced in the nineteenth century to stop wind-driven rain from penetrating to the inside surfaces of a building. The main reason for building cavity walls has always been to keep the rain out, but it was found that the air layer trapped in the cavity also provided a degree of thermal insulation.

To increase this thermal insulation Building Regulations since the 1980’s 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 the last few years, there has been a drive to “retro fill” cavity walls of houses built before 1980 with materials blown into the cavity.

Since this started there have been an increasing number of complaints about cavities filled with blown mineral-wool fibre. This consists of loose fibres blown through holes drilled in the brick outer leaf. The manufacturers and installers claim that the material is water-repellent, and that it cannot allow rainwater to penetrate across the cavity. This appears to be incorrect and the fibres can soak up water like blotting paper.

There are several consequences of this effect:

  1. Inside walls have become damp and mouldy, and homes have become uninhabitable and unsellable. People experiencing “increased condensation” since CWI was installed do not realise that the source of the extra water running down their windows and dripping from their ceilings is rainwater penetrating via the cavity wall insulation, and so have not bothered to report it to anyone.
  2. Following CWI some people have not seen savings on fuel costs, in-fact some have seen bills rise and their homes feel colder. This is because blown mineral-wool fibre only has to become 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 heat out of the house.
  3. Research has shown that 40 percent of houses whose cavity walls have been filled with blown mineral fibre suffer from gaps in the insulation. These voids can cause condensation and black mould on the walls especially near ground level, between windows, and at ceiling level in upstairs bedrooms.
  4. Another problem is insulation sinking to the bottom of the cavity, leaving cold areas at the top of the house or below windows, 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, but who are insulating up to five houses per day at a price of only £100 each, when a good professional job should take a whole day, and cost at least £500. (If you have had blown fibre cavity insulation, and you suspect that it has settled, leaving gaps, then you can check for this by drilling holes near the top of the bedroom walls and inserting a fishing weight on a piece of string).
  5. Even when a cavity has been properly filled with mineral-wool fibre, the material will still break down and collapse over time. The fibres themselves become brittle and the material simply compacts under its own weight. How long does this take? – nobody knows, because the problem has never been independently researched.

Installers and their “guarantee” provider CIGA (the Cavity Insulation Guarantee Agency) refuse to acknowledge that the cavity wall insulation was the cause of the problems mentioned above. They insist that the dampness problems were due to construction faults in the building or to “lifestyle condensation” caused by the occupants (even though condensation had not been a problem prior to the CWI).

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. They state that: “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, a reduced thermal performance and damage to internal finishes.”

The insulation installers are supposed to do a thorough survey of the cavity, and proceed only if the wall meets strict British Standards. In practice, hardly any cavity walls meet these standards, as they all have unfilled mortar joints, debris dropped down the cavity, and wall-ties covered with mortar droppings. But the installers press on regardless, and these imperfections and obstructions catch the insulation and stop it from filling the cavity evenly.

There are no statistics about this, because no research has been done, however the problem may be widespread, and under-reported.

Infra-red imaging company IRT Surveys Ltd has surveyed 250,000 properties across the UK and found that one-third of homes are well insulated, one-third have no insulation at all, and one-third have damp, slumping or missing insulation. The third with no insulation at all will include all the solid-walled Georgian, Victorian and Edwardian houses. The other two-thirds will be predominantly cavity-walled properties, and these statistics indicate that HALF of these are likely to have faulty insulation. CIGA itself has issued six million guarantees, so if IRT’s findings hold true, this would indicate that some three million UK homes have current or potential CWI problems.

Recently some homeowners have taken their cases to litigation and won the argument, and even more significantly the subject has been raised in parliament. Is this the insulation industries ‘PPI’ type disaster?!

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