Is cavity wall insulation a good idea?

If you have cavity walls and they haven't been insulated, up to a third of the heat produced in your home could be escaping. Insulation should reduce your heating costs and carbon emissions from your home significantly. The insulation itself needs to be suitable for the conditions inside a masonry cavity, and so choices are limited to three options: blown mineral wool, plastic beads or plastic foam.

According to the Energy Saving Trust, cavity wall insulation should cost from £350 to £700 (depending on the house of your home) and save between £100 and £250 per year - so can pay for itself in about 3 years. The carbon saving will be between 400kg and 1 tonne per year. This makes it about five times better than installing a solar PV roof in terms of both carbon savings per pound spent and payback time.

The installation process must include an assessment to ensure that the construction is of a suitable type. Installers should work through one of the schemes mentioned below, and guarantee the installation for 25 years. It may take about 3 hours to inject the insulating material into the cavity. Some homes may be classified as 'hard to fill' cavities - perhaps too narrow or uneven to fill easily. Insulation may then be more expensive to install (perhaps 2 to 3 times as much), or it may not be feasible - external or internal insulation may then be better.

A Which? Magazine survey in 2011 found that some installers were not undertaking adequate assessments. According to industry guidelines, they should inspect all external walls thoroughly to check for cracks/defects, check all internal walls to check for any existing damp, and do a cavity check (with a drill hole) on all walls. A proper survey like this is likely to take an hour. Do ask a few companies around to give quotes.

Cavity wall insulation should not cause problems of dampness, but a proper assessment of the property is needed to ensure it would be suitable. If your home is towards the west coast of the UK, more prone to wind-driven rain, then it might be unsuitable for cavity insulation if it's in a very exposed, unsheltered position and there are cracks in the external wall. For those few homes in this position, measures could taken to prevent damp risks - for example by putting extra protection in the form of  boards or tiles on the exposed walls. There will be a cost to this of course, and it should be compared with quotes for options such as internal or external wall insulation.

Some BRE (Building Research Establishment) research in the 1990s showed that cavity wall insulation when assessed & installed properly does not lead to an increased risk of damp. The study found that the structural condition of the walls was the most important factor in damp problems - for example, badly filled mortar joints or 'dirty cavities' (where, during construction, mortar has dropped down inside the cavity - i.e. if too much is used). Over time, this can cause problems with damp-proofing.

Contacts

British Board of Agrément - http://www.bbacerts.co.uk - 01923 665300
  Tests & approves building materials and issues certificates to verify performance and compliance with UK Building Regulations. Also run approved installer schemes for insulation & other building work.
Cavity Insulation Guarantee Agency (CIGA) - http://www.ciga.co.uk - 01525 853300
  Provides 25 year guarantees for cavity wall insulation if fitted by registered installers - a list of these is on the web site.
National Insulation Association - http://www.nia-uk.org/consumer/ - 08451 636363
  Trade Association. List of members on the website, e.g. for finding people to undertake cavity wall, loft, or solid wall insulation (internal & external), or for doing draught-proofing work.

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