Cavity wall insulation – complete guide

What is cavity wall insulation?

Cavity wall insulation is a method of improving the thermal efficiency of buildings constructed with cavity walls – two layers of masonry (an inner and an outer wall) separated by a small air gap. This gap, known as the cavity, can allow heat to escape easily if left uninsulated.
By filling the cavity with an insulating material such as mineral wool, polystyrene beads, or foam, homeowners can significantly reduce heat loss, lower energy bills, and improve indoor comfort throughout the year.

Cavity wall insulation is one of the most effective and affordable ways to increase a property’s energy performance, especially in older homes built before the 1990s.

Common materials used for cavity wall insulation include:

Mineral wool (glass or rock wool) – excellent thermal and acoustic properties
Polystyrene beads or granules – lightweight and moisture-resistant
Urea formaldehyde foam – used mainly in older installations

How does cavity wall insulation work?

The insulating material is injected or blown into the wall cavity, creating a thermal barrier that slows down the movement of heat. This prevents warm air from escaping during winter and keeps heat out during summer months. Over time, this leads to reduced energy consumption, lower carbon emissions, and improved living comfort.

Benefits of cavity wall insulation

Cavity wall insulation offers a wide range of long-term advantages:

Benefit	Description
Reduced energy bills	Less heat escapes, meaning your heating system runs more efficiently and consumes less energy.
Improved comfort	Maintains stable indoor temperatures all year round.
Environmental impact	Lower energy consumption helps reduce CO₂ emissions.
Increased property value	Homes with insulation have higher EPC ratings, making them more attractive to buyers.
Noise reduction	Certain insulation materials also absorb sound, reducing external noise.

Cavity wall insulation cost

The cost of cavity wall insulation varies depending on the property size, type of insulation material, and installation company. On average, homeowners in the UK can expect to pay:

Property type	Average cost (approx.)
Apartment / flat	£500 – £800
Semi-detached house	£900 – £1,200
Detached house	£1,200 – £2,000

While this may seem like a significant investment, the annual savings on energy bills can range between £200 and £400, allowing the insulation to pay for itself within a few years.

Cavity wall insulation grants

The UK government and local councils occasionally offer cavity wall insulation grants as part of national energy efficiency schemes.
These grants are typically aimed at:

Low-income households
Pensioners or those receiving certain benefits
Homeowners looking to reduce their carbon footprint

Some of the schemes to look out for include the ECO4 scheme (Energy Company Obligation) and regional sustainability programmes.

It’s always worth checking the latest updates on the UK government’s website or contacting an approved installer to see if you qualify for funding.

Common cavity wall insulation problems

While cavity wall insulation provides many benefits, it’s not suitable for every building. Incorrect installation or poor material choice can lead to potential problems, such as:

Moisture penetration: in properties with damaged brickwork or exposure to driving rain, insulation may trap moisture.
Cold bridging: if parts of the wall remain uninsulated, heat can still escape through these sections.
Material degradation: over time, some insulation materials can settle or deteriorate, reducing efficiency.
Condensation or damp issues: improperly installed insulation may affect ventilation and lead to damp patches inside walls.

Before installing, a qualified surveyor should always assess whether your property is suitable for cavity wall insulation.

Cavity wall insulation removal

In cases where insulation has been incorrectly installed or caused damp problems, cavity wall insulation removal may be necessary.
The process involves extracting the insulation material from the cavity using specialised vacuum systems. This is usually done when:

The insulation material has degraded
Damp or mould has developed inside the property
The house is located in an exposed or coastal area

After removal, the cavity can be refilled with a more suitable insulation type if appropriate.

Cavity wall insulation issues to watch for

If you already have cavity wall insulation, keep an eye out for these common issues:

Damp spots appearing on interior walls
Musty smells or mould growth
Increased heating bills without a clear reason
Cold patches in certain rooms

If any of these occur, contact a professional for a thermal imaging survey or inspection to determine whether the insulation is performing as it should.

Is cavity wall insulation right for your home?

Not every property is a good candidate for cavity wall insulation. It’s most suitable for:

Homes built after 1920 with unfilled cavity walls
Properties in areas with low rainfall or good external wall condition
Buildings with accessible wall cavities at least 50mm wide

If your home is older or solid-walled, external wall insulation (EWI) or internal wall insulation (IWI) may be a more effective option.
You can learn more about these systems in our external wall insulation section on our website.

Penguin BM – your insulation experts in London

At Penguin BM, based in Wembley, London, we specialise in external wall insulation systems and offer expert advice on all types of wall insulation.
Although we primarily focus on EWI systems, we provide educational resources to help homeowners understand the differences between insulation methods, their costs, and long-term benefits.

You can also visit our online store to explore a range of professional products for external wall insulation, including adhesives, insulation boards, mesh, and finishing systems from trusted brands such as Lakma and Tytan.

Summary table – cavity wall insulation overview

Aspect	Details
Purpose	To reduce heat loss through the gap between two masonry walls
Common materials	Mineral wool, polystyrene beads, foam
Average cost	£500–£2,000 depending on property size
Potential problems	Moisture, damp, cold bridging
Removal	Possible using specialised extraction systems
Alternative solution	External wall insulation (EWI)
Ideal for	Post-1920s cavity wall homes

Cavity wall insulation remains a popular and cost-effective way to improve home energy efficiency. However, it’s not a one-size-fits-all solution. Always consult an experienced professional before installation to ensure your property is suitable. If you’re considering improving your home’s insulation, explore other energy-saving systems such as external wall insulation, which offers added protection and aesthetic benefits.