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Cavity Wall Insulation

If you have a house with cavity walls it can make a real difference to have cavity wall insulation installed.  Cavity wall insulation was installed in our own house some years ago, but now we are seeing most of our neighbours installing it too.  This makes sense because the normal cost of installation would be £500, but with the current grants available it can cost half this amount.  There are some people who can get cavity wall insulation installed for free, based on their age or whether they are on benefits.  The latest information on eligibility for grants can be found on the Energy Saving Trust website, www.energysavingtrust.org.uk.

There can be an issue if you live in a flat because you will have a shared cavity wall.  However, this can be overcome if your neighbour(s) who share the cavity also want to install cavity wall insulation.  Our neighbours did this recently as they had a shared cavity between their ground and first floor flats; they were both able to get a grant and the installer was happy to install the insulation in both properties together.

The estimated fuel bill savings following installation are £115 a year, based on 3-bed semi-detached house with gas central heating.  The reduction in heating bills will depend on how you use your heating currently, but what my neighbours report is that their house feels warmer than it did, using the same heating settings, prior to installing cavity wall insulation.  What this means is that if you have been putting up with a cold house to keep your bills down, it will now be more comfortable.  Or if you had high energy bills trying to keep your house warm you should now see a reduction in your energy bills.

Why is this so?  There is always a flow of heat from a warmer temperature area to a colder temperature area, e.g. from inside a house to the outside.  This does not rely on open windows or doors; this movement takes place through the walls all the time.  The presence of a cavity between the inner and outer walls reduces this movement, insulating that cavity reduces it further. 

One way of expressing this improvement is to look at the U-value.  The U-value is the heat flow density (watts for each square meter, W/m2) for each one degree temperature difference between the air inside and the air outside.

Technical Information

Our house was built in the early 1970’s with a brick cavity wall; giving it a U-value of 1.6 W/m2K

When the cavity wall was filled this gave it a U-value of 0.5 W/m2K

What this means is that the house with the unfilled cavity is losing around three times as much heat as the house with the filled cavity.  This can be calculated, e.g. on a cold day, say an outdoor temperature of 5ºC, when the inside temperature is 20ºC:

• the heat flow rate for 10 m2 of unfilled cavity wall will be 224 W from inside to outside;
• the heat flow rate for 10 m2 of filled cavity wall will be 70 W from inside to outside.