Cavity Wall Insulation

Most homes built after 1920 have cavity walls. These houses have two layers of brickwork, seperated by a gap. This gap is called the cavity, and it’s responsible for heat loss. If your home doesn’t have cavity wall insulation, heat will pass through the air in the cavity and escape from the house.

An operative in a blue hard hat placing a layer of fibre wool cavity wall insulation into the cavity of a newbuild house

Cavity Wall Insulation (CWI)

Filling the cavity with insulation beads, foam, or wool stops heat from travelling across the walls. You can think of the insulation like a thermal vest between your skin and a jumper – an extra layer which keeps the heat in.

Firstly, the installer drills small holes into the outer wall. Next, they pump the insulation into the cavity. Once the cavity is full, the installer re-fills the any holes in the outer wall.

Cavity wall insulation materials have improved over time. If your insulation is old, you may need to replace it. Contact the installer and ask them to check the condition of the insulation. If the installer cannot help, check the contact details on the guarantee. The installer should have supplied this when they completed the work. Your home report often contains any work guarantees.

Is my home right for cavity wall insulation?

Firstly, you’ll need to check that your home has cavity walls. If your home is less than 100 years old, it’s likely to be the right type of construction.

Your Energy Performance Certificate will tell you what type of walls your home has. If they’re cavity, it will also tell you whether they are already insulated.

Another way to tell whether you have cavity walls is by looking at your home from the outside. Where possible, check the brickwork to see how the bricks have been laid. If all the bricks look the same length, then the wall is likely to have a cavity. If the bricks alternate between short and long, then the wall is unlikely to have a cavity.

Once you’ve established that your home has cavity walls, an installer would need to check that:

  • The cavity is at least 50mm wide
  • The wall is in good condition
  • There’s no rubble in the cavity
  • The walls aren’t exposed to driving rain


Cost of cavity wall insulation

Cavity wall insulation typically costs around £20 per m2, making it the most cost-effective kind of wall insulation. The exact cost will vary depending on the insulation materials.

Regardless of the size of your home, cavity wall insulation should pay for itself within five years. This is because the amount you’ll save on your energy bills over five years should equal the cost of the insulation.

It’s possible to receive grant funding for cavity wall insulation. For more information, see our Grant funding for cavity wall insulation section below.

Types of CWI

There are several different kinds of materials for insulating cavity walls. These wall cavity fillers vary in cost, effectiveness and how environmentally friendly they are.

You may be familiar with mineral wool already – it’s commonly used in loft insulation and is sold in big yellow rolls.  

Mineral wool is made in a similar way to candyfloss: a machine spins molten rock so that the fibres weave together. 

The installer blows tufts of mineral wool into the cavity. These tufts knit together, forming an insulating layer. 

Mineral wool is relatively cheap and effective. You can also recycle it.

The installer blows polystyrene beads into the cavity. Once the beads are inside the cavity, they expand and bond together, ensuring good coverage. Like blown mineral wool, polystyrene beads are relatively cheap whilst also being very effective.

The installer sprays polyurethane foam into the cavity. When the insulation foam dries, it solidifies. Whilst polyureathane foam is incredibly effective, it is also about twice the price of mineral wool or polystyrene bead insulation.

Removal of cavity wall insulation

If cavity wall insulation is installed properly, there shouldn’t be any need to remove it. However, in the 1980s many installers used the wrong materials or applied cavity wall insulation to unsuitable properties.

Incorrectly installed insulation may not maintain the warmth of your home. It can also cause issues with condensation and damp.

In cases such as these, a professional should remove the cavity wall insulation.

The company carrying out the cavity wall insulation removal will:

  • Redrill the holes in the outer wall
  • Remove a brick from the bottom of the outer wall
  • Use a vacuum to suck out the insulation from the hole in the bottom of the wall
  • Pump air through the original holes to blow the insulation towards the vacuum. This fully clears the cavity.

Cost of cavity wall insulation removal

The average cost of cavity wall insulation removal is £20 – £25 per m2. This varies depending on the type of cavity wall insulation material.

You might be able to avoid paying for cavity wall insulation removal. If the installer made mistakes during the installation or initial survey, then they should arrange to fix the problem or remove the insulation for you. You should ask the installer to visit your home and check that everything was carried out correctly.

If the installer can’t help or has gone out of business, the work may still be covered by a CIGA guarantee. A standard CIGA guarantee is 25 years. You can contact CIGA to check whether your insulation is still under guarantee.

If you don’t have a CIGA guarantee, check to see if the work is covered by any other kind of guarantee.

The work may have taken place before you moved in to your home. In that case, check the Home Report for any guarantees.

If neither CIGA nor the installer can help, you may need to pay a company to remove the insulation for you. We’d recommend getting at least three different quotes to make sure you’re getting the best price. You can use the National Insulation Associations’ search tool to find a company in your area.

Cavity Wall Insulation FAQs

Yes, as long as all the flats above and below yours get the insulation as well. The cavity runs the entire height of your block, so you can’t fill just one section of it. The installer pumps the insulaton in at the top of the building, so it fills up from the bottom. You can think of it like pouring marbles into a pint glass. It’s impossible to just fill the top third or middle third of the glass – you have to fill the entire thing from the bottom. 

There has been a lot of misinformation about cavity wall insulation causing damp. Cavity wall insulation in itself is very unlikely to result in damp. The vast majority of issues are due to bad practice on the part of installers.  

This is why it’s important to always use an accredited insulation company. A responsible installer will make sure: 

  •  Not to block any ventilation (such as vents and airbricks)
  •  To address any pre-existing issues with damp before installing any insulation
  •  The cavity is free of rubble or debris 
  •  The exterior walls are in good condition 
  •  To check if your property receives high levels of wind-driven rain. 

Cavity wall insulation is installed from the outside, so there’s no need for installers to enter your home. 

Depending on the height of your home, the installer may need to use scaffolding. This will definitely be the case if you live in a flat. 

The installer will drill a number of holes into the outer wall, then pump in the insulation. There will be noise from both the drilling and the pump. Afterwards, the installer re-fills the holes.

Grant funding for cavity wall insulation 

Grant funding for cavity wall insulation is available through our Home Energy Scotland service.  

The Home Energy Scotland Grant offers homeowners up to £1,500 towards cavity wall insulation. On top of that, £500 is available through an optional interest-free loan. 

To apply for Home Energy Scotland Grant funding, simply call their freephone number on 0808 808 2282 to speak to an advisor.

Register your interest in our retrofit service

Register your interest in EcoCosi from Changeworks to find out more about how to retrofit your home and to be sent information about the service.

Home Energy Scotland logo

Changeworks delivers Home Energy Scotland in the south east and Highlands and Islands on behalf of the Scottish Government and Energy Saving Trust.

As well as providing free, impartial expert advice to thousands of people every month to help them to keep warm in their homes for less, they identify funding opportunities for households seeking to install energy efficiency measures.

For more information, give Home Energy Scotland a call on 0808 808 2282 or email and the team will be happy to help you.