Cavity Wall Insulation

Cavity Wall Insulation Explained

Many cavity walls can be insulated by injecting insulation material into the cavity from the outside. A specialist company will drill holes in the outside walls, inject insulation through the holes and then seal them with cement. The insulation material is usually either mineral wool or polystyrene beads, but polyurethane foam may sometimes be used instead.

To insulate your cavity walls, the installer drills small holes around 22mm in size at intervals of around 1m in the outside wall of your home. The installer then blows insulation into the cavity using special equipment. Once all the insulation is in, the installer fills the holes in the brickwork so you’ll barely notice them.

Filling cavity walls is not a job you can do yourself, you will need to employ a registered installer. A professional can do the job in around two hours for an average house with easily accessible walls. It shouldn’t make any mess.

Costs and savings

Typical installation costs of cavity wall insulation vary depending on the size of your home. But whether you live in a large detached house or small flat, you should be able to make back the installation cost in five years or less due to the yearly energy bill savings you will make. As a rough guide, £800 is what you can expect to pay to install cavity wall insulation in a detached house. Annual energy savings could be as much as £200/ year on a detached home (EST).

You might be able to reduce these costs by carrying out the work at the same time as other home improvements or by not tackling the whole house at once.

Is cavity wall insulation right for your home?

If your house was built in the last 20 years or so, the walls are probably already insulated. To find out whether they are, you can do the following:

  • ask a registered installer for a borescope inspection. The installer will drill a small hole in your external wall to see if your walls are hollow or filled
  • check with your local authority’s building control department

Your home will be suitable for standard cavity wall insulation if it meets the following criteria:

  • its external walls are unfilled cavity walls
  • your cavity is at least 50mm wide, and is clear of rubble
  • the masonry or brickwork of your property is in good condition
  • the walls are not exposed to driving rain
  • your house is not at risk of flooding

You will need an installer to carry out a survey to check that your house is suitable. If so, they will then be able to insulate your walls using mineral wool or polystyrene beads.

If your house has narrow or uneven cavities, is in an exposed site or there is a risk of flooding, then it may be possible to fill the cavity with polyurethane foam. This is more expensive than standard cavity wall insulation but is a particularly effective insulator. You will need a specialist foam insulation installer to survey your home for this, and to carry out the work if suitable.

If you have any damp patches on your internal walls then they should not be insulated until the problem is resolved. Speak to a builder who specialises in damp prevention.

If your home’s external walls are joined to another house, the installer will need to insert a cavity barrier to contain the insulation so your neighbours aren’t affected.


The installer should be a member of one of the following organisations:

Check whether the installer is signed up to a code of professional practice and that the installation is guaranteed for 25 years by CIGA, or through an independent insurance-backed guarantee.

Solid Wall Insulation

Insulating your solid walls could cut your heating costs considerably and make your home more comfortable.

If your home was built before the 1920s, its external walls are probably solid walls rather than cavity walls.

  • solid walls have no gap, so they can’t be filled with cavity wall insulation
  • cavity walls are made of two layers with a small gap or ‘cavity’ between them

Solid walls can be insulated though – either from the inside or the outside. This will cost more than insulating a standard cavity wall, but the savings on your heating bills will be bigger too.

Internal Wall Insulation

Internal wall insulation is done by fitting rigid insulation boards to the wall, or by building a stud wall filled in with insulation material such as mineral wool fibre.

  • is generally cheaper to install than external wall insulation
  • will slightly reduce the floor area of any rooms in which it is applied (the thickness of the insulation is around 100mm)
  • can be quite disruptive, but can be done room by room
  • requires skirting boards, door frames and external fittings to be removed and reattached
  • can make it hard to fix heavy items to inside walls – although special fixings are available
  • Cannot be done before fixing any problems with penetrating or rising damp

External wall insulation

This involves fixing a layer of insulation material to the wall, then covering it with a special type of render (plasterwork) or cladding.

The finish can be smooth, textured, painted, tiled, panelled, pebble-dashed, or finished with brick slips.

External insulation:

  • can be applied without disruption to the household does not reduce the floor area of your home
  • will renew the appearance of outer walls
  • will improve weatherproofing and sound resistance
  • fills cracks and gaps in the brickwork, which will reduce draughts
  • increases the lifespan of your walls by protecting the brickwork
  • reduces condensation on internal walls and can help prevent damp (but will not solve rising or penetration damp)
  • best installed at the same time as external refurbishment work to reduce the cost
  • may need planning permission – check with your local council
  • requires good access to the outer walls
  • not recommended if the outer walls are structurally unsound and cannot be repaired

Moisture Movement and Ventilation

In traditionally built properties with solid walls, water vapour can usually move quite freely through the building. This is partly because of the high levels of ventilation and draughts, but also because water vapour can travel through the bricks and stones that the walls are made of. When you insulate an older building, you will change the way that water vapour behaves in several ways:

  • adding wall insulation will usually cut down on draughts through the walls and round the windows
  • the insulation may create a barrier to vapour movement, depending on what materials are used
  • adding insulation to the inside of a wall will make the wall colder. This means that any water vapour entering the wall from inside will get a lot colder, and may condense inside the wall

Whenever you fit solid wall insulation to a building you need to take account of water vapour to make sure that you don’t create new damp problems in the future. This may involve using “breathable” insulation materials that will allow the vapour to carry on permeating the walls, or it could involve creating a continuous vapour barrier to make sure no vapour can get into the walls from the inside. You will need an experienced specialist installer to develop a moisture control strategy that is specific to your building.

You should check with your installer that the installation is covered by an appropriate 25 year guarantee. They may be members of the SWIGA guarantee scheme, or they may offer an independent insurance-backed guarantee. You can find a list of Ofgem approved guarantee schemes.

Typical installation costs* of solid wall insulation can vary as follows:

external wall insulation: around £10,000

internal wall insulation: around £8,200

*Based on a typical semi-detached house in Great Britain

Over 20 years payback period for a gas heated UK detached home. However, off gas grid is more expensive to heat and therefore annual savings will be greater.

The costs we suggest for installing solid wall insulation are for paying a company to come in, insulate your whole house in one go, fully redecorate and replace everything just as it was.

If you’re looking to spend less, it is advisable to insulate a wall when you are having other building or decorating work done. Internal insulation can be fitted when you’re planning to redecorate anyway, or to fit a new kitchen or bathroom. You can also spread the cost by tackling one room at a time.

External insulation will also cost less if you fit it when you’re having other work done to the outside. If you’re having a new roof, or painting the windows, or even having solar PV panels fitted, then you will probably have scaffolding up already, which can save a bit on the costs. If your walls need repointing or other repair work, it’s worth getting a quote for a complete refurbishment including insulation – it will probably work out cheaper than doing the two things separately.

Sometimes there is financial support available to help with the cost of solid wall insulation.

Certified Installers:

We recommend you use a Solid Wall Insulation Guarantee Agency (SWIGA) certified installer for internal and external wall insulation. This may also be important for securing funding. See more about SWIGA and the up to date list of certified installers on their website here.

Financial help:

HEEPS ABS – Interior insulation grants for eligible households

Warmwowrks – all insulation types for eligible households

Home Energy Scotland – all insulation types loans and grants

Energy Company Obligation (ECO3) – all insulations for eligible households