A question I am often asked at exhibition seminars is should cavity walls have cavity wall insulation. Almost invariably the answer is yes. The typical cavity is 50mm (2 inches) wide and 50mm of foam insulation will reduce the heat loss through the wall by around 75%. As 35 per cent of the total heat lost from a house is through the walls, that gives an overall saving of 28 per cent.

An average three to four bedroom house, around 150m² floor area is likely to have a heating bill of about £700 per year (assuming a condensing gas boiler). Cavity wall insulation will save about £200 per year (and reduce carbon emissions by 2.5 tonnes per year).

Getting the cavities filled is likely to cost around £500, but remember the CERT scheme. This is Carbon Emissions Reduction Target, imposed on energy companies by the Government. All the major energy companies (British Gas, Npower, Scottish & Southern, EON and EDF) are required to provide support to householders to reduce the carbon emissions, which support comes in the form of grants.

If you are over 70 years old, on benefits or disabled you can get the walls and loft insulated free. Even if you are none of the above you can still get a grant of up to 50 per cent. Any of the energy companies will do a free survey and tell you what needs to be done, if cavity wall insulation will work and how much it will cost (or you can ring your local Energy Advice Centre on 0800 512 012 to see what grants are available in your area).

When NOT to insulate
Some properties, typically built before 1940, have no vertical damp proof course (VDPC) around the windows. They rely on the ventilation in the cavity to stop rainwater penetrating to the inside wall. In this case you will need to either insert a VDPC or not fill the cavity.

In any house, cavity wall insulation will reduce ventilation, preventing natural moisture build-up from being removed. So if you have cavity wall insulation make sure you also have trickle vents or extractor fans.

What NOT to use
Cavity wall installers’ favourite material is mineral wool because it’s cheap and it’s very quick to blow in. Gangs of installers tend to do lots of houses every day, and often do them poorly. This leads to problems with voids in the insulation, where they have missed bits out, and slumping, where the mineral wool was not packed in tight enough to hold in position. Both these will lead to cold spots, reducing efficiency and increasing the potential for condensation in the cavity.

My advice about cavity wall insulation is do it, but make sure you do it right. Mineral wool is the worst option for this work and I would always prefer cellulose beads. If I can’t afford that I would go for polystyrene beads or injected foam.

photo by Elsie esq.

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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