Cavity wall insulation: Everything you need to know

How does cavity wall insulation work?

A cavity wall has two walls held together by wall ties with a gap between them. The gap is known as the cavity. The gap prevents rain from getting into the property through the outer wall.

Cavity wall insulation works by storing heat within the inner walls before bouncing it back into the room and retaining it for longer.

The result? A warmer home with less wasted energy!

Because heat does not travel through the air very easily, it’s a natural insulator. The materials used to insulate cavity walls per m² are chosen because they trap air between their fibres, creating what’s known as an ‘air matrix’. This means that insulated walls retain heat far more effectively than empty cavity walls.

How will my cavity walls be insulated?

If you have cavity walls, the gaps in your walls can be insulated by injecting material in from the outside.

Your trader will check that the walls are suitable before ensuring they’re in good condition and that you don’t have any issues with damp.

If everything is in order, holes will be drilled in from the outside, at various points along the wall, and insulation is then injected through these holes. Once the walls have been filled, the holes are sealed over with cement.

How do I know if I have cavity walls?

Properties in the UK have either solid walls or cavity walls. As the name suggests, you can only have cavity wall insulation if you have cavity walls.

So, if you’re thinking of getting cavity wall insulation, the first thing you’ll need to do is work out if your property has cavity walls.

The age of your home is a great place to start. As a general rule, properties with cavity walls started being built in the 1920s and were a popular technique until building regulations changed in the mid-1980s.

Another clue is the appearance of your walls. If you have cavity walls, the bricks will usually be arranged in an even pattern, and all the bricks will be laid lengthways. If your bricks are arranged in an alternating pattern, producing rows of what looks like long and short bricks, chances are your walls are solid.

Of course, a professional trader will be able to tell you for certain whether your property has cavity walls. They can check whether or not the walls are solid with what’s known as a borescope inspection, which involves drilling a small hole in your wall. Alternatively, your local authority’s building control department might be able to help.

The benefits of having cavity wall insulation in your UK home

There is a wide range of benefits associated with cavity wall insulation. It’s one of the most cost-effective home improvements out there.

Lower energy bills

With cavity wall insulation, the savings you will make on your energy bills are significant. So much so, that the cost of the work usually pays for itself in less than five years.

A warmer home

A third of all heat that escapes from your home does so through the walls. Cavity wall insulation helps to prevent it.

Reduced carbon footprint

When your heating escapes through the walls, you’ll need to use more energy to heat your home. Cavity wall insulation prevents the escape of energy, meaning you’ll use less energy and, as a result, lower your carbon footprint.

Quick and easy installation

Although you’ll need a professional to carry out the work for you, the actual process of installing cavity wall insulation is relatively quick, easy and disruption-free. It can normally be completed within two hours, depending on the size of your property, of course.

Types of cavity wall insulation

There are several different types of cavity wall insulation. The most popular and common materials to use for this type of insulation are:

Blown mineral fibre

Blown mineral fibre is one of the most common types of cavity wall insulation. It consists of strands of fibreglass or mineral wool, which are then forced into the wall cavity using compressed air. These fibres then expand to fill out the space.

Mineral fibre is similar to the quilt insulation used in lofts. For walls, it is broken down into small tufts to allow it to be blown in.

This is a relatively cost-effective insulation method, and it’s easy to install. As a result, it is often used by free cavity wall insulation schemes.

However, it’s not without its downsides. Blown mineral fibre can only be used for standard sized or wide cavities as, if it’s used in cavities under 50mm, there is a risk of it becoming patchy. It’s also important that it is correctly installed to avoid problems further down the line.

Mineral fibre must be kept totally dry to avoid it losing its insulating properties.

This type of insulation is covered by the British Board of Agrément (BBA) Certificates and can be used anywhere in the country.

Polystyrene beads and granules

EPS beads and granules are blown into the cavity using compressed air with a mix of adhesive. When the adhesive cures, it bonds the beads together and creates an insulated barrier within the wall cavity.

The beads can be supplied loose or in a light resin to hold them together. Or, if you’re using granules, these will stick together on their own, thanks to their shape.

This type of insulation is usually used in walls with a narrower cavity. It is very effective at trapping heat and creating really solid cavity wall insulation free from gaps. Another benefit of this type of insulation is that it allows any moisture to drain through to the ground, helping to prevent damp.

However, it has been known for loose granules to escape through the airbricks, particularly if you have work done on your property at a later date that requires the walls to be cut or drilled into.

Polystyrene beads are covered by the British Board of Agréments (BBA) Certificates.

On Checkatrade, you'll only find trades who meet our high standards and pass up to 12 checks.

Foam

This type of insulation is pumped into the cavity. The foam is created instantly within the wall cavity when two chemical components are injected and mixed.

Polyurethane foam can be injected into the wall cavity through small holes, achieving results that should last as long as the life of the building without any further maintenance.

The best type of cavity wall insulation for your property will depend on several things, including how thick your wall cavities are, the degree of humidity and the atmosphere in your property, and your budget.

How much does cavity wall insulation cost to install? 

You’re best off talking to your local cavity wall insulation experts to get an accurate quote.

However, the UK average cost to install cavity wall insulation can be found below.

As a general rule, you can expect to pay around £30 - £65 per m². This is based on polyurethane foam.

Check out our cavity wall insulation cost guide for more information.