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Cavity wall insulation

Insulating your cavity walls is a simple and effective way to reduce your heating bills – and have a warmer home 

If your home was built after the 1930s, the chances are that its external walls are made of two ‘skins’ with a small gap between them. This means they are ‘cavity walls’ and the gap between them can be filled with insulating material to stop the warmth escaping to the outside.

To find out whether your home has cavity walls, look at the brick work. If the bricks of your walls show only their long sides (as in this photo) your home has cavity walls; if they show an alternating long-short-long-short pattern (like in this photo) then your home has solid walls.

Cavity wall insulation can cut your heating bills so much that you'll very quickly recover the costs

Cavity wall insulation can cut your heating bills so much that it will pay for itself quickly. What’s more, there are grants and offers available to help pay for cavity wall insulation.

For professional installers, the job is simple, quick (about two hours) and makes no mess.

Is my home suitable?

Generally speaking, your home will be suitable for cavity wall insulation if:

  • The external walls are unfilled cavity walls
  • The masonry and/or brick work is in good condition
  • Your cavity is at least 35mm wide (or 50mm for some forms of insulation)

But where the following situations apply, and where no remedial action is appropriate, cavity wall insulation should not be installed.

  • Poor maintenance or structural issues like cracks in the walls
  • Non traditional construction 
  • Cavity widths of less than 50mm. These are often categorised as ‘hard-to-treat’ cavities for which insulation systems do exist
  • Dirty cavities, dirty wall-ties or builder’s rubble in the cavity
  • Exposed locations, particularly where the outside walls are exposed to extreme weather and driving rain
  • Overflowing or blocked guttering

Some walls can also be unsuitable for cavity wall insulation. In all cases if you’re thinking about cavity wall insulation, always check with a registered installer who will assess whether your home ticks all the right boxes. If your home is not suitable for cavity wall insulation, why not think about external or internal solid wall insulation?

How is it installed?

The most common way of insulating cavity walls is by drilling a series of small holes, each around 25mm across (about the diameter of a ten pence-piece) into the mortar at regular intervals and injecting the insulation material into the cavity. The holes are then refilled once the insulation has been blown in. Some installers make use of a telescopic lance system for walls that are hard to access. New-build properties often have rigid insulation boards fitted into the wall cavities as part of the construction process.

Which material should I use?

The most common material used in cavity wall insulation these days is polystyrene 'beads'. Those in the image below have a carbon coating for extra performance.

Polystyrene beads are more popular than alternative materials – mineral fibre and expanding foam – because they are more effective at keeping the heat in, and don't absorb moisture. Beads are also quicker to install as they can be blown easily and far into the cavity, reaching awkward areas (e.g. where there is a conservatory, shed or car port) without the need to drill lots of holes in the wall. This means there's less patching up to do afterwards and also no need to use scaffolding which adds to the cost of the work.

How much money will I save?

We estimate that savings for a typical gas-heated, 3-bed semi are as follows (correct as of April 2013).

Annual saving£190
Payback timearound 3 years
CO2 saving per year831kg

For larger homes, or those heated by electricity or oil, the savings will be significantly higher.

Finance options for energy efficiency improvements like cavity wall insulation may be available to you. Click here for more information.

NB: Filling cavity walls is not a DIY job. It should always be carried out by a registered installer who is a member of one of the following professional bodies:

You should also make sure that your installer is signed up to a code of professional practice like those provided by the NIA, and that the installation is guaranteed for 25 years by CIGA.

Won’t cavity wall insulation cause damp?

Many people, including some building professionals, believe that wall cavities should never be filled and that insulating cavity walls will inevitably lead to damp bridging. This is simply not true.

To date, around 4m UK homes have had cavity wall insulation retro-fitted. The number of cases where cavity wall insulation has directly caused damp bridging is very small indeed. Where water penetration does occur this is usually due to defects in the construction of the property and not due to the faulty or inappropriate installation of cavity wall insulation.

Defects, like cracks or pre-existing damp problems can easily be spotted by a competent assessor and can usually be remedied, meaning that the walls can be insulated and damp problems avoided. This is one reason why it is so important to have a proper assessment carried out before any insulation is put in.

Guaranteed for 25 years

Any reputable installer will be registered with the Cavity Insulation Guarantee Agency (CIGA). The CIGA guarantee is for 25 years and covers problems that are caused by the faulty or inappropriate installation of cavity wall insulation. If you have cavity wall insulation already and are worried that there is a problem with it then you can contact CIGA to check whether your guarantee is still in place.

Where a defect in materials or workmanship is involved CIGA always seeks to resolve complaints to the customer's complete satisfaction. CIGA will have your property registered if the cavity walls have been insulated by an installer covered by the guarantee.

Frequently asked questions

How can I tell if I have cavity walls?

There are three simple things you can check. Firstly, the age of your house. If it was constructed before 1932 it is unlikely to have cavity walls. If it dates from 1932-1982 it probably does, and after 1982 almost certainly. Secondly, check the pattern of the brickwork. If you see only the long side of the brick ('stretcher bond' in brickie parlance) the property is likely to have cavity walls. If you see a long-short-long-short pattern, this suggests a solid wall with no cavity. Finally, measure the thickness of your wall. If it is more than 30cm (11.5 inches) and constructed of brick it is likely to have a cavity, although some stone walls can be considerably thicker than this.

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

Contact us Or freephone: 0800 082 2234

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Can all cavity walls be insulated?

Most cavity walls can be insulated, but there are a few exceptions including those where the masonry or brick work of the property is in poor condition, the wall-ties are dirty, the cavity is less than 50mm wide or if the walls are exposed to persistent driving rain (e.g. coastal properties)

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

Contact us Or freephone: 0800 082 2234

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View all frequently asked questions

What materials are used for cavity wall insulation?

CWI comes in three main types. Expanding foam, mineral fibre and the increasingly widely used poly-bead or carbon bead which tend to be the most energy efficient.

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

Contact us Or freephone: 0800 082 2234

Next question

View all frequently asked questions

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This information is available as a freely downloadable PDF from this page.

For more domestic energy advice, view all our advice pages.

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