Readers letters
PE (8 Questions survey, November 2011) reported that about a half of the engineers surveyed had replaced their loft or cavity wall insulation in the past five years, and more in the past 10 years. I have some concerns about post-construction cavity wall insulation. My starting point is an extract from the Building Regulations Summary, taken from its website:
“Resistance to moisture: Contaminants, weather and ground moisture should not be allowed to get into a building. Normal construction using such elements as damp proof courses, damp proof membranes, lead flashings, cavity walls and suspended floors reduce moisture penetration into buildings. Linings and specialised controls can protect building occupants from potentially hazardous substances in the ground, such as lead, arsenic, radon and methane.”
Cavity wall construction evolved in the latter part of the 19th century as a solution to damp penetration arising from poor standards of construction. It would have been laughable, then, if a cavity fill of whatever was suggested. The ventilated cavity was the solution.
New builds with insulation panels placed in the cavity during construction, and which maintain a cavity, are probably the best solution. If the gap is filled with material, a cavity no longer exists, and it seems to me that its moisture transmission prevention purpose could be compromised.
Installers' websites that I have looked at claim that the (ubiquitous) blown fibre used will not transmit moisture. If so, then the function of such a wall in preventing the penetration is changed from that of a ventilated cavity to (ideally) the equivalent of an impervious membrane. I say ideally, because in practice I'm not convinced that the fill achieved is homogenous. Time and thermal cycling will surely cause settlement, permitting moisture transport across wall ties, or wicking from water running down the inside surface of the outer brick leaf.
If serious water ingress occurred, a house would become unsaleable. Installers’ guarantees cover only defects in the cavity filling process, not the rebuilding of walls. In answer to a question from me about responsibility for consequences, the government advised that the installers would carry out a survey and assessment to see if the property was suitable for cavity filling. An ill-advised practice – the hunter advising the quarry.
Perhaps some engineers who have retrofitted cavity wall insulation could report whether they are satisfied that it was worthwhile.
Terry Bryant, Weaverham, Cheshire
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