Cavity wall insulation

Cavity wall insulation is polystyrene, mineral wool or foam that is packed into brick walls which have a narrow gap – or cavity – between them. It helps prevent heat escaping from your home.

If your home has walls with a cavity between them, a layer of insulation can be inserted inside it.

The process involves drilling small holes in the outer wall, through which the material is injected in liquid form. It will then harden between the walls and the holes will be filled in again with cement.

The job should be carried out by a specialist insulation installer and – depending on the size of the property – typically takes less than a day.

There are several ways to check if your walls are solid or have a cavity:

• The thickness of the wall. If your wall is more than 260mm thick (measure this at an outside door or window) it probably has a cavity. The cavity is likely to comprise an outer layer of brick and an inner layer of either brick or large concrete blocks
• The brickwork. If your walls have a cavity, the outer brickwork will form an even pattern with every brick laid lengthways. If you have solid walls, some bricks in the pattern may have been laid endways.
• The date it was built. If your home was built post-1920, it is likely to have cavity walls.

Once you’ve established your walls have a cavity, how can you tell if they have already been insulated?

If the property was built after the 1990, it has probably had cavity wall insulation already built in.

If it’s an older property, check if there’s an outstanding energy performance certificate against it. These are free to access and download at the government’s official register – just search by address.

The EPC will state ‘filled cavity’ if the walls of the property have been insulated. If the walls haven’t been inspected, but the age and type of the property would suggest they should have insulation, it will say ‘assumed’.

If you are buying or renting a new home, you will be given an EPC from the seller or landlord – so ask them for more information on the cavity walls as well as any paperwork.

Alternatively, you could commission a borescope inspection. This involves an installer drilling a small hole into your external wall using a tiny camera to see if it’s hollow or filled.

Or check with your local authority's building control department.

Not all homes are suited to cavity wall insulation. Here are some potential common blockers:

• The wall cavity is too narrow: The gap between the walls need to be at least 50mm wide
• Access is limited: The installer will need to be able to reach all the external walls
• The cavity walls are already filled: You will need a clean, empty space into which cavity wall insulation can be fitted
• The brickwork is in bad condition: It will need to be repaired before cavity wall insulation can be installed
• The walls are exposed to damp or driving rain, or at risk of flooding: Cavity wall insulation could exasperate the problem and cause damp – more on this below
• You live in a flat: In this case, you will need an agreement from the managing agent or freeholder – as well as other residents in the block

Cavity wall insulation installed in unsuitable homes – for example, those with old or poor brickwork, in the face of driving rain or at risk of flooding – can cause damp.

Water is not able to escape from the cavity, the insulation becomes damp and there is no means of ventilation for it to dry out again.

In this case it may be possible to fill the cavity with polyurethane foam – a more expensive material than standard cavity wall insulation. Contact a specialist foam insulation installer to survey your home and see if it’s suitable.

Cavity wall insulation could also cause damp if it’s been badly installed or unevenly distributed. Use a member of the National Insulation Association, the Cavity Insulation Guarantee Agency or the British Board of Agrément as they offer a 25-year CIGA guarantee for their work and materials. Members of these groups should also tell you if your home is not suitable for cavity wall insulation.

It’s designed to last the life of the building, but the Cavity Insulation Guarantee Agency (CIGA) runs for 25 years.

The cost of cavity wall insulation varies according to the type of property – but you can expect to pay about £725 to insulate a detached house, or about £475 for a semi-detached property, according to the Energy Saving Trust. 

Some energy providers offer free cavity wall and loft insulation to customers who own their own home and are in receipt of certain benefits such as pension credit, universal credit or disability living allowance. Check your supplier’s website or give them a call to find out if you might qualify.

Keep an eye out too for more details on the government’s Green Homes Grant scheme, which is due to launch in September 2020.

The scheme will offer homeowners in England vouchers of up to £5,000 to put towards the cost of carrying out energy efficient improvements – including cavity wall and loft insulation. 

Cavity wall insulation is a great way of retaining the heat in your home and in turn, saving on energy costs. But there are many other ways too – as we set out in our energy saving tips.

Perhaps the most easy and effective way to save money on your energy bills is to compare deals with MoneySuperMarket.

Simply answer a few questions about yourself, your property, and your approximate energy usage, and we’ll return a list of quotes. These will be from a range of providers including the Big Six and a range of smaller firms.

When you’ve found the best one for you, we’ll take you through to the provider to finalise the deal.

If your new energy supplier has signed up to the Energy Switch Guarantee scheme it has pledged to do the following:

• Handle the entire switching process from start to finish
• Ensure you are not without energy at any point during the switch
• Resolve any problems swiftly and efficiently
• Guarantee the switch in 21 days
• Allow you to change your mind in the first 14 days and remain with your current provider

All the Big Six are members of the scheme as well as many smaller providers. Check the full list here.