Cavity Wall Insulation

Key takeaways

• Cavity wall insulation is a thermal barrier that fills the gap between two brick walls in a house, preventing heat loss

• Not all homes are suitable for cavity wall insulation, especially those with narrow cavities, poor brickwork, or exposure to damp

• Cavity wall insulation costs around £4,600 for a detached house and £2,700 for a semi-detached. But can substantially cut your energy bills over time

• Annual savings on your heating bills from installing cavity wall insulation range from £100-£370 per year, depending on your property

What is cavity wall insulation?

In essence, energy-efficient cavity wall insulation is a thermal barrier that fills the void between two brick walls in a house.

This gap, if left uninsulated, can be a significant source of heat loss. To combat this, various materials such as polystyrene beads, mineral wool, or foam are used to fill this cavity, effectively trapping heat inside your home.

How does cavity wall insulation work?

Specialists begin by drilling small holes into the external wall of your home. Through these, they inject the insulation material in a liquid form that expands and hardens, creating a solid thermal barrier.

Once the cavity is filled, the holes are sealed with cement, and the entire installation typically takes less than a day.

Identifying cavity walls

Before you consider insulation, it's crucial to determine whether your home has cavity walls or solid walls. Here are a few pointers:

• Wall thickness: Measure the thickness at a door or window. A thickness greater than 260mm usually indicates a cavity

• Brickwork pattern: An even pattern with bricks laid lengthways often means there's a cavity

• Construction date: Homes built after 1920 are likely to have cavity walls

Do you already have insulation?

For properties constructed post-1990, built-in insulation is common. However, to be certain, you can:

• Look for an energy performance certificate (EPC) on the government’s official register – it's free and searchable by address.

• The EPC will list 'filled cavity' if insulation is present.

• Alternatively, a borescope inspection or a query with the local authority's building control department can offer confirmation.

Suitability for cavity wall insulation

Not all homes are ideal candidates for this type of insulation. Factors that might render a property unsuitable include:

• Narrow cavities (less than 50mm)

• Limited access to external walls

• Existing insulation

• Poor brickwork condition

• Exposure to elements like damp or driving rain

• Tenure agreements in flats

Does cavity wall insulation cause damp?

Cavity wall insulation installed in unsuitable homes – for example, those with old or poor brickwork or at risk of flooding – can cause damp.

This is because water is not able to escape from the cavity, the insulation becomes damp and there is no means of ventilation for it to dry out again.

In this case it may be possible to fill the cavity with polyurethane foam – a more expensive material than standard cavity wall insulation. Contact a specialist foam insulation installer to survey your home and see if it’s suitable.

Cavity wall insulation could also cause damp if it’s been badly installed or unevenly distributed.

Use a member of the National Insulation Association, the Cavity Insulation Guarantee Agency or the British Board of Agrément as they offer a 25-year CIGA guarantee for their work and materials.

Members of these groups should also tell you if your home is not suitable for cavity wall insulation.

Longevity and cost

Cavity wall insulation is designed to last the lifetime of your building, backed by a 25-year guarantee from the Cavity Insulation Guarantee Agency (CIGA).

The cost varies depending on the property type but expect to pay around £4,600 for a detached house and £2,700 for a semi-detached, as per the Energy Saving Trust.

The average cost for a mid-terrace home is £1,500.

Can I get free cavity wall insulation?

For those on certain benefits, free insulation might be available from energy providers.

Check with your provider to see if they're part of the Energy Company Obligation (ECO) scheme, a government energy efficiency scheme that aims to tackle fuel poverty and cut carbon emissions.

Cavity wall insulation vs loft insulation

Cavity wall insulation is usually significantly more expensive to install than loft insulation.

Figures, using data from June 2024, from the Energy Saving Trust indicate that while cavity wall insulation could set you back £1,500 for a mid-terrace house, loft insulation would typically cost £900 for the same property.

But what about the savings? That's a closer run thing. The Energy Saving Trust claims that you can expect to save about £180 per year by installing 270mm of loft insulation in a mid-terrace house that had no loft insulation previously.

If you were to add install cavity insulation in the same home, you could expect to save £130 per year.

How else can I keep down the cost of my energy bills?

Cavity wall insulation is a great way of retaining the heat in your home and in turn, saving on energy costs. But there are many other ways too – as we set out in our energy saving tips.

From comparing energy deals to understanding your usage, there are numerous strategies to keep your bills down.

The Energy Switch Guarantee Scheme

This scheme is a commitment to a hassle-free switch between energy suppliers. It promises:

• A smooth transition process

• No interruption to your energy supply

• Swift resolution of any issues

• Completion of the switch typically within five working days

• A 14-day cooling-off period

Both major and smaller energy providers participate in this scheme, ensuring peace of mind when you decide to switch.