Cavity wall insulation: what it is, how much it costs and what to consider

With energy prices set to hit new highs for at least the next two years, we reveal what you need to know about cavity wall insulation, including how much it costs and the potential savings.

Why it's important to use less energy

The Government is set to roll out two massive financial packages next month to help households deal with rocketing energy bills.

Typical energy bills looked set to hit an eye-watering £3,549 in October - three times higher than in February last year.

In a bid to avoid financial catastrophe for millions, the Prime Minister has announced that average annual bills will instead be frozen at £2,500 from next month, which will last for a total of two years.

On top of that, most homes will also receive a one-off £400 payment to help cover the high usage winter months.

While this will help ease the burden significantly, it still means energy bills are going to be at a record high for the foreseeable future. 

In the past, the obvious thing to do in order to slash your energy bills was to switch to a cheaper tariff. Nowadays that will have minimal, if any benefit, as there really aren't any cheap options out there. 

As a result, the best way to save is to simply use less energy while minimising any discomfort. 

One possible way to achieve this is through cavity wall insulation, which is affordable to most households and can have a notable impact on your winter energy bills.

In this guide, we'll look at everything you need to know about cavity wall insulation, including the costs, potential savings, and any important considerations.

What is cavity wall insulation and how does it work?

According to Energy Saving Trust, around a third of all heat lost in an uninsulated home escapes through the walls, which is why insulating them can help slash your heating bill.

If you live in a house that was built after the 1980s, your property is likely to have wall insulation fitted already, but if your home is older it may have cavity walls.

A cavity wall consists of two walls with a gap in between – the outer layer is usually made of brick while the inner layer is made of brick or concrete block according to Energy Saving Trust.

Alternatively, you might have solid walls with no cavity. These cannot be filled with cavity wall insulation but you can insulate them from the inside or outside.

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Is cavity wall insulation right for your home?

Your home will usually be suitable for cavity wall insulation if:

• The external walls are unfilled cavity walls;
• The cavity is at least 50 millimetres wide;
• The walls are not exposed to driving rain;
• The masonry or brickwork of your property is in a good condition;
• Your house is not at risk of flooding.

You should have a survey before deciding on cavity wall insulation to make sure this is right for you.

During a survey, the surveyor will check whether your home is suitable for cavity wall insulation. They might drill a few holes to check the width of the cavity and whether it is free of debris as well as if there are any issues with damp.

The surveyor may recommend a specific insulation material based on these factors.

It’s worth finding an installer registered with CIGA as you’ll have a 25-year guarantee for any cavity wall insulation. All member installers of CIGA must follow technical guidance and the independent body’s best practice guide.

Poor cavity wall insulation can cause damp and mould issues, while ventilation problems can become worse, so it’s vital that you choose a good installer.

If you live in a flat, you won’t be able to only have your flat insulated according to Energy Saving Trust as you’ll usually need everyone to agree to insulate the whole block.

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How is cavity wall insulation installed?

If you choose to get cavity wall insulation, a specialist installer will drill holes around 22 millimetres in size at intervals of around one metre into the outside walls, inject insulation and then seal these holes.  

It's a good idea to choose a registered installer who will typically take around two hours (for a house with easy-to-access walls) to install cavity wall insulation.

There are different insulation materials that can be used, and this can affect how much you pay – we’ll explore your options later in this guide.

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How much does cavity wall insulation cost and how much can you save?

The cost of cavity wall insulation can vary depending on the size of your property and the materials used.

Energy Saving Trust estimates it can cost between £410 for a mid-floor flat and £2,500 for a detached property. These estimates are based on fuel prices as of April 2022 and may change from October.

It’s worth stressing that these are average costs as the size of your walls can impact how much you pay – so the bigger your house is, the more it’ll cost.

It’s also worth warning that any quotes are valid for a set amount of time (usually at least a month) and costs can go up if you need to get another quote.

While cavity wall insulation can be expensive, it is guaranteed for 25 years if you use a registered installer with The Cavity Insulation Guarantee Agency (CIGA).

On average, you can save £285 a year on energy bills for a semi-detached house or £480 on average for a detached property, so the savings will grow over many years.

Can I get free cavity wall insulation?

You may be eligible for free cavity wall insulation from your energy supplier if you qualify for certain benefits and own your property (or in some cases rent from a private landlord).

A quick way to find out if you can get free cavity wall insulation is to try the Simple Energy Advice questionnaire, while you can find out which energy firms offer insulation for free under the ECO scheme here.

You should then contact a participating energy supplier to find out whether free cavity wall insulation is available in your area and whether you’re eligible.

You’ll need to get a survey to make sure that cavity wall insulation is suitable for your home.

Our friends at MoneySavingExpert warn that, while the work should be free, there might be a charge if it’s a complex installation, but you can choose not to go ahead with it.

What materials are used for cavity wall insulation?

There are several different materials that can be used for cavity wall insulation, including wool, polystyrene beads, or foam.

Mineral wool insulation can be used in cavity walls and is cheap and easy to install.

But wool insulation can only be used for standard or wide cavities as it can be ‘patchy’ in cavities of less than 50 millimetres, according to energy saving advice site The Green Age.

Alternatively, the installer can inject small balls of polystyrene insulation that expand in the cavity wall, resists moisture and can be used in difficult areas. It’s also quick to install.

If you have uneven cavity walls, foam insulation might be an option but it’s more expensive. The foam is injected into the wall where it forms a barrier that also repels water.

According to TradesmenCosts, you can expect to pay £13-£18 per square metre for glass wool insulation, while polystyrene beads will set you back between £18 and £22 per square metre.

Polystyrene foam insulation is the most expensive as it costs on average between £22 and £26 per square metre.  

How to find an installer for cavity wall insulation

As many issues can be caused by poorly fitted (or unsuitable) cavity wall insulation, it’s vital to find an installer that’ll do a good job.

CIGA offers a 25-year guarantee for any cavity wall insulation although consumer champion Which? also recommends choosing a member of the National Insulation Association (NIA) or the British Board of Agrément (BBA).

What should I do if I have any issues?

If you have any issues with your cavity wall insulation, contact the installer as soon as possible.

If the company is registered with CIGA, the installer should get in touch with you within 24 hours to investigate what’s happened.

CIGA says if the installer hasn’t resolved the issue within 41 working days, then you should contact CIGA to tell them what the issue is, and any steps taken so far.

They’ll arrange an inspection of the property, compile a report and recommendations, and if necessary, carry out remedial work. You can find out more details about CIGA’s complaints process here.

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