Cavity Wall Insulation

What is a cavity wall?

A cavity wall is made up of two masonry brick walls running parallel to one another with a space (cavity) between them of at least 50mm. Masonry bricks are very absorbent, so moisture absorbed by the outer wall typically drains through the cavity, rather than coming into the home, helping to prevent damp issues. This type of wall construction became the norm in the 1930s superseding solid walls and as time has gone on, the size of the cavity between the two skins of brick has continued to grow – a typical cavity wall now is between 280-300mm thick.

Although some builders began insulating cavity walls in the late 1970s, it only became compulsory under building regulations to do so during the 90s. As such there are many properties in the UK that currently have unfilled cavity walls. The good news it that these can be insulated very easily!


A home can lose as much as 35% of its heat through uninsulated external walls. Therefore if these can be insulated, the home retains heat better, which means lower energy bills.  The concept of insulating a cavity wall is really very simple – it involves filling the cavity between the two skins of masonry bricks with an insulating material, which slows the movement of heat through the wall.

How does cavity wall insulation work?

If a hot room is partitioned from the cold by a wall, heat will move through the wall, eventually cooling the room until an equilibrium is reached, where the outside temperature is equal to the inside temperature. In reality this very rarely happens, because rooms tend to be heated, so as some heat escapes through the wall, more hot air is supplied, keeping it at a comfortable ambient temperature. If the thermal gradient is larger, (e.g. on a cold and wintry day), the movement of thermal energy across the wall will be accelerated.

Insulating a cavity wall helps to provide a thermal barrier, which slows the flow of heat out of a room considerably. By slowing down the rate at which heat escapes from the home, less heating is needed to keep the house at the required temperature. In the summer, the reverse happens; hot air outside the home can’t get in as easily, which means you don’t need to use energy to keep the home cool. Therefore in both summer and winter, cavity wall insulation can make an enormous difference to your energy bills. The process is relatively quick and inexpensive, so it is certainly worth considering.

Could you benefit from cavity wall insulation?

Stretcher bond

Cavity Wall – Stretcher Bond

There are two things you need to determine to see whether you can benefit from retrofitting cavity wall insulation in your home.

The first thing is to work out if you actually have cavity walls – this might seem stupid, but you can not inject insulation if there isn’t a cavity and they do look quite similar to solid walls!

You can easily identify a cavity wall by the pattern produced by the brickwork, which is known as stretcher bond, where are the bricks are running in the same direction as one another – there are no ‘half bricks’. This is obviously harder to do if your walls are cladded or painted and in this case you might need to call in a professional (although sometimes you can see original brickwork in the loft space). In addition cavity walls tend to be over 250mm in width, with more recent cavity walls closer to 300mm. If you can see lots of half bricks in your wall, you have a solid wall with no cavity, so unfortunately cavity wall insulation is a no-go.

Once you have established that you have cavity walls, you need to determine the size of the cavity and whether it has previously been insulated. A registered installer will need to come and carry out a boroscope inspection. This involves drilling a test hole into the wall and checking with a camera to see if the cavity has previously been filled and the size of the cavity (ideally over 50mm). If this shows the cavity is unfilled, you could indeed benefit from the cavity wall insulation. If you would like us to come and carry out an inspection on your walls, please fill in the form below.

How do you insulate cavity walls?

The first thing to note is that you cannot retrofit cavity wall insulation as a do-it-yourself job – it is a job that needs to be carried out by a professional.

Once the cavity has been confirmed by the boroscopic inspection, the installer will drill a series of 22mm diameter holes into the mortar between the bricks. With specialist equipment, the installer will then inject the cavity with the insulating material, through each of these holes. Once the whole of the cavity wall has been filled, the mortar will be made good either with plugs or mortar created to match the existing colour, so the job will be barely noticeable.

Cavity walls being Injected

The insulating material pumped into the cavity is normally a type of glass wool, or in some instances insulating beads and once installed will offer insulation for the life of the building. The whole process should only take about 2 hours but obviously if the cavity wall area is especially large you will need to leave more time for the job to be completed.

Cavity wall insulation is not always suitable!

In some instances, cavity wall insulation might not be suitable, especially if your property is exposed to driving rain. Installing cavity wall insulation in this instance can actually create damp issues on the inside of the property, since the cavity can now no longer drain the water.

Unfortunately, insulation sometimes still does get installed on these properties – the reason for this is mainly because the installers have been installing under Government schemes so want to maximise installs to maximise profits. Unfortunately the initial survey should have picked up that cavity wall insulation was not suitable, but 9 times out of 10 this doesn’t happen.

If this has happened to your property, and you need advice it is worth getting in touch with our partners at extraction4homes to help get this removed – they will look at the best way forward in terms of resolving the issue (this can sometimes be covered by the CIGA warranty). If the insulation does need to be removed and you still wish to improve the thermal comfort of your property then you will need to look at a different insulation solution like internal wall insulation.

In addition, if there are already damp issues on the inside of the property, these need to be sorted before insulation is installed, otherwise the insulation could compound the damp issue. In this instance, a builder with expertise in damp prevention will need to assess the situation.

The final issue relates to access; if the cavity wall cannot easily be accessed by the installation team (and their equipment), then you might also need to look at another insulation solution.

Savings from cavity wall insulation

Although the savings from cavity wall insulation vary greatly from property to property, for an average size three bedroom home, the energy savings from installing cavity wall insulation should amount to £250 per year. With an installation cost of £600-1000, the savings you create from installing the cavity wall insulation should pay for the work in under 4 years.

Paying for cavity wall insulation

Unfortunately schemes like the Green Deal have now finished, but there is still some ECO funding for cavity wall insulation. The amount of funding depends on the heat demand of the property and the savings that will be generated from installing the insulation. Over time the level of funding has decreased dramatically, so it is now quite normal that a household contribution may need to be required – for example if the cost of the job is £1000, you may be required to fund half of this so £500.

If you are interested in getting cavity wall insulation installed, we have a directory of ECO funded cavity wall installers, so please fill in the form at the bottom of this page.


    • Insulating your cavity walls will help you to heat your home more efficiently, saving about £250 for a typical 3 bed home.
    • Cavity wall insulation will payback in 3 – 4 years for the investment giving lower heating bills .
    • According to the Energy Saving Trust, cavity wall installation can reduce carbon dioxide emissions (CO2) by 560kg, according to the Energy Saving Trust.
    • Approved cavity wall installation work is guaranteed for 25 years by the CIGA (Cavity Insulation Guarantee Agency).


    • Cavity wall insulation may not be suitable within your home, if it has a wall exposed to strong rainy wind.
    • Do not undertake the installation if the home suffers from damp problems – seek an assessment from a professional surveyor first


    • Cavity wall insulation can cost anywhere between £600 – 1000 (however with subsidies, the cost may end up at the lower end of this estimate – speak to your Energy provider).


Want cavity wall insulation in your property?

Interested in getting cavity wall insulation?

Fill in the form below and we will be in touch!

Are you Human? *

* Required Fields