In this article

What is a cavity wall? 

What is cavity wall insulation? 

How to tell if the walls in your home have cavities 

How to check if your home already has cavity wall insulation 

Is cavity wall insulation suitable for your home?

The cost of cavity wall insulation 

Saving money with cavity wall insulation

Carbon footprint savings

How to install cavity wall insulation

Other ways to insulate the home 


Cavity wall insulation is one of the easiest and most cost-effective forms of insulation to install in the home. It increases the energy efficiency of your home, reduces heat loss and your carbon footprint, and can make the home warmer and cheaper to heat. Most older homes built from 1920 onwards will have cavity walls, and unless the home was built in the last 15-20 years, it is likely that they will be hollow, resulting in greater heat loss.

Filling the cavity wall with insulation is a great way to save on energy bills and could save you up to £305 per year.1 Read on to learn everything there is to know about cavity wall insulation, the benefits and the different types available to help you make a smart choice.


What is a cavity wall?

Most homes built from 1920 onwards will typically have been built with them, and it’s probably a term you’ve heard before, but what exactly is a cavity wall?  

A cavity wall is a type of wall that is built in a home and contains a hollow centre. This hollow centre is known as the ‘cavity’. This cavity has one wall either side, one on the inside of the home and one on the outside, leaving a small space in the middle. This cavity can either be empty or filled with insulation – kind of like the filling in a sandwich. 

The two walls are known as the inner and outer walls and are often referred to as ‘leaves’ or ‘skins’. They are typically made of concrete blocks or bricks, with the most popular types of materials being aggregate or clay.  


What is cavity wall insulation? 

This refers to any insulation that is installed in a cavity wall, the hollow centre between the inner and outer walls of the home. This type of insulation is installed by drilling holes into the external wall, through the mortar joint. The holes are typically 22-25mm, the size of a small ring. Once the holes are drilled, insulation is injected into them to fill up the cavity. The holes are ‘made good’ once installation is complete, leaving no mess. 


How to tell if the walls in your home have cavities 

The first thing to find out is the age of your home, and this may be available with a quick search on the internet, checking your mortgage survey or checking HM Land Registry. If your home was built after 1920, it’s likely that you will have a cavity wall or solid wall construction.   

You should check for any visible or exposed brickwork on the outside of the home. If you find that all the bricks are laid lengthways, are equal in length and have a longer outline, this will typically mean that a cavity wall construction has been used. If the bricks aren’t equal in length, with some placed ‘end on’, it is likely that you have a solid wall construction. In any case, the installer will undertake a full assessment and be able to confirm if your home is suitable, and what kind of construction you have.  

You can see below how a typical cavity wall construction appears when being looked at from the outside of the home, note the bricks are all laid lengthways and equal in length. 

Photograph of exterior wall of home showing cavity wall construction


How to check if your home already has cavity wall insulation 

Depending on the age of the home, it may already have insulation installed in the cavity wall. Most homes built after 1990 will typically have their wall cavities insulated during the construction of the property. 

For older homes, the cavity wall may be empty and suitable to insulate, in either case, we recommend following this advice to find out for sure: 

Check your Energy Performance Certificate (EPC): The EPC should also tell you whether cavity wall insulation has been installed. You can search here to find whether your property has one.

Check with your local authority: There will be a record on file with the local building control department of any previous installs of insulation. It is worth checking with them first – prior to booking a borescope inspection. 

Ask a registered installer: They will be able to drill a small hole in the wall and let you know whether the cavity wall is empty, or has insulation installed already. This is known as a borescope inspection. 

Check the exterior walls of the property: Once installation is complete, any holes drilled will be filled in, but some faint marks may be left on the external walls, so keep a look out for these. Remember, they will be the size of a small ring.


Is cavity wall insulation suitable for your home? 

Once you’ve established if cavity walls are used in the construction of the home, there are some other key points to consider. Cavity wall insulation is only suitable if: 

  • Your home currently has unfilled cavity walls which are made of brick. The answer to this can be found out through one of the options described above. 

  • The cavity is at least 50mm wide, and clear of any rubble. 

  • The brickwork and masonry of the property is in good condition.  

  • The walls aren’t exposed to heavy rain.  

  • The home isn’t located in a high-risk flooding area. 

  • The internal walls are dry and not exposed to wet conditions, or damp. Wet insulation is ineffective so if this is an issue, you will need to get this sorted prior to install.  

  • There is no steel or timber frame construction used in the home.  

If you meet all these criteria, cavity wall insulation may be suitable for you. We know it may be difficult to answer all of these for certain, so you should always speak with a reputable installer. The installer will be able to complete a full assessment to determine the suitability of the home and give you peace of mind that you’re making the best choice for insulation. 

Note that if some of the walls are connected to a neighbour's home, the installer will need to use a cavity wall barrier, which can increase the cost of installation.   


The cost of cavity wall insulation 

Insulating cavity walls is one of the cheaper types of insulation and comes together with some great savings on annual energy bills. The installation does need to be carried out by a professional installer, but even with these costs factored in, the type of insulation is still cost-effective.  

The cost will vary depending on the size of the home, but the key thing to note is that all types of home, from a detached home to a one-bedroom flat will experience energy bill savings and recoup the initial cost of installation within 5 years at most. These are the typical costs that you can expect to pay for cavity wall insulation, by type of home:2

  • Detached: £610 

  • Semi detached: £475 

  • Mid terrace: £390 

  • Bungalow: £460 

  • Flat: £345 

Recoup your costs in just two years:

To give you an example of how quickly you could recoup your initial investment for installation, take a detached home which has an installation cost of £610. For this type of home, the estimated savings on heating are £305 per year – meaning you will recoup your install costs in just two years.3

Government grants 

One area to explore when looking to install insulation is to consider any Government heating and home improvement grants that are available. With the Green Homes Grant having now been withdrawn, it does leave homeowners with less choice for funding, but there is still one scheme available. We recommend looking at the Energy Company Obligation (ECO), which is a means tested scheme designed to help with the costs of install when certain criteria are met, such as receiving certain benefits. While this scheme is not open to everyone, you may find that you could be eligible.  


Saving money with cavity wall insulation

In addition to having a warmer, more comfortable home, cavity wall insulation allows you to benefit financially with reduced annual energy bills resulting from having a properly insulated home. Your installation costs can be recouped within a couple of years, meaning the savings thereafter are yours to spend how you wish, typical annual energy bill savings are:4

  • Detached: £305 (£/year) 

  • Semi detached: £180 (£/year) 

  • Mid terrace: £115 (£/year) 

  • Bungalow: £120 (£/year) 

  • Flat: £90 (£/year) 

Insulation can be done at the same time as other works on the property, which can reduce callout fees and labour costs. For example, if you are considering switching to a low-carbon heating system such as an air source heat pump or hybrid heating, then having good insulation is essential prior to install. Your Energy Performance Certificate (EPC) may have already highlighted insulation as a suggested improvement in the home. 


Carbon footprint savings 

Installing cavity wall insulation is a great way to reduce your carbon footprint and help fight climate change, by creating a more energy-efficient home. More heat will be retained in the home meaning the heating system doesn’t have to work as hard or be on for as long to reach the optimum temperature, generating less carbon emissions in the process. 

You could reduce your carbon footprint by this much annually just by installing cavity wall insulation:5

  • Detached: 1.5 tonnes (co2/year) 

  • Semi detached: 920kg (co2/year) 

  • Mid terrace: 580kg (co2/year) 

  • Bungalow: 620kg (co2/year) 

  • Flat: 455kg (co2/year) 

So, as you can see, installing cavity wall insulation can benefit both your wallet and the planet as well. To give you an idea of just how much carbon you could save, the detached home example above of 1.5 tonnes of carbon emissions per year, is the equivalent of driving nearly 4,000 miles in an average passenger vehicle – quite the saving indeed! 


How to install cavity wall insulation 

While some forms of insulation can be installed as a DIY job, such as roof and loft insulation or draught proofing, cavity wall insulation is best left a professional installer. The reason for this is that prior experience, training, and specialist equipment are needed to ensure a high-quality install.  

The install process 

  1. Checking suitability: Any good installer will typically assess the home and look for any existing cavity wall insulation using a borescope inspection. They’ll check the suitability criteria we mentioned earlier are met including the general condition of the walls and looking for any signs of damp. After this assessment, they’ll let you know whether cavity wall insulation is suitable. 

  2. Drilling of holes: Once the home is confirmed as being suitable, the installation can start. A series of small holes (between 22-25mm in diameter) are drilled along the exterior wall, which are typically placed at one metre intervals.  

  3. Injection of insulation: The installer injects the insulation material through the holes into the cavity using specialist equipment, filling the cavity and creating an additional layer between the inside and outside of the home. The material used here will either be foam or beads.  

  4. Making good: Once the installation is complete, which usually takes around 2 hours, the installer will fill the holes in the brickwork with cement and clear away any mess. You may see a faint outline from where the holes have been drilled, but this will be subtle.  

After the installation 

With the installation now complete, you’ll be able to experience the benefits immediately. Your installation will likely be guaranteed by the Cavity Installation Guarantee Agency (CIGA) for 25 years, and you’ll be sent a certificate shortly after the install is complete.  

If you do experience problems with your walls or insulation following the install (such as damp), then your installer should be the first point of contact. They should work with you to establish a solution, and in most cases will be able to fix any issues that may occur.  

In the event that you find them unwilling to help, you can contact CIGA directly to make a claim under the 25-year warranty. It is worth asking the installer during the quote process to double check the install will come with a CIGA warranty, as it is not compulsory. 

Insulation material 

There are three main types of material used for cavity wall insulation. All of these follow the same process of being blown into the holes drilled in the exterior wall of the home:  

  • Blown mineral fibre: These are strands of fibre glass or mineral wool, which is blown into the cavity using compressed air. This is the most common material for installs. 

  • Polystyrene beads: These small beads are very effective at trapping heat and creating a gap free installation. The small beads are pumped into the cavity using compressed air and used to fill up the cavity. 

  • Foam: The third type of material that can be used is formaldehyde foam. Two chemicals are mixed and injected into the drilled holes. These chemicals then expand to fill the cavity. It offers the best thermal insulation but is tricky to install, so many suppliers opt to use mineral fibre or beads.  

How to find a good installer 

Cavity walls need to be insulated by a professional, so this means you will need to have a look around for a reputable installer to carry out the work. There are organisations and safeguards in place to protect homeowners, and your installer should be a member of one of these organisations: 

  • The National Insulation Association (NIA) 

  • The Cavity Insulation Guarantee Agency (CIGA) 

  • The British Board of Agreement (BBA) 

These websites are a great first place to start looking for a reliable installer, they even have an ‘installer search’ to find installers in the local area. By choosing an installer who is registered with these organisations, you can be sure that your install is guaranteed.  

In addition to finding an installer who is a member of these organisations, make sure you follow these tips as well: 

  • Get 2-3 quotes: The costs section of this guide should give you a rough idea of how much cavity wall insulation costs, but it’s always good to get 2-3 different quotes from local suppliers. They should be able to give you a full quote and will typically offer a no-obligation assessment of the home. 

  • Look at reviews: Previous customer experiences are usually telling of how good an installer is. Check reviews for your installer on Trustpilot, trader sites such as Check-a-Trade and their social media. 

  • Ask friends or neighbours: Know a friend or neighbour who has recently had insulation installed? Why not ask them about their experience? 


Other ways to insulate the home 

In addition to cavity wall insulation, there are lots of other options to insulate different areas of the home, you can read all about these in our ultimate guide to home insulation. Here are a few to note:

Roof and loft insulation

Heat escapes from all areas of the home, but the roof and loft are one of the areas which experiences the greatest heat loss. In fact, 25% of heat is lost through the roof of an insulated home, so it’s one important area to consider to the energy efficiency of a home reduce heat loss and can save you up to £275 per year on energy bills.6

Solid wall insulation

If solid walls are installed in the home, they too can be insulated. Solid walls contribute to 45% of overall heat loss in the home, so insulating them can be a great way to reduce heat loss.7 Typical energy bill savings can be up to £375 for a detached home, so it’s a worthwhile investment.8

Draught proofing

Draught proofing is one of the cheaper options to keep the home warm and efficient. This work can usually be done as a DIY project. Draught proofing includes sealing gaps in doors, skirting boards and walls. Typical savings for draught proofing are around £20 per year.9


1,2,3,4,5,6,8,9 Source: https://energysavingtrust.org.uk/advice/cavity-wall-insulation/

7 Source: https://www.energyagency.org.uk/en/solid-wall-insulation_46988/