What Is the Best Cavity Wall Insulation?

best cavity wall insulation

Nowadays, it’s rare to find a modern house without proper cavity wall insulation because of UK building regulations. These regulations are crucial for comfortable living and energy efficiency. Unfortunately, some older houses might not have insulation or lack sufficient insulation. Additionally, poorly-installed insulation might need replacement. This begs the question: what’s the best cavity wall insulation?

There are numerous types of insulation materials available depending on your requirements and preference. For example, you might find the more environmentally-friendly perlite the most suitable insulation option, or you might prefer our recommended choice, foam insulation.

Do you want to find out more? In this article, we’ll tell you everything you need to know and how to choose the most suitable cavity wall insulation material for your situation. 

What Is Cavity Wall Insulation?

Cavity wall insulation is a process where you add a poor-conducting material to the wall cavity space to prevent heat loss. Typically, this happens during the construction of new buildings. However, if you live in an older house or your home has sustained damage over time, you might need to have the wall cavities filled.

The way this insulation works is by filling the space between internal and external walls with an insulating material, trapping the air that would previously circulate and conduct heat. The air in the wall prevents heat convection, so, theoretically, the trapped air in the wall cavity acts as the insulation.

Why Is Insulating Cavity Walls Important?

The answer is simple: to preserve heat and save . If you don’t want to spend a fortune on heating bills, but also want to be comfortable in your home, cavity wall insulation is the answer.

Cavity wall insulation has become a stable part of energy efficiency in UK housing, so much so that the government is offering free insulation grants to encourage households to have their homes insulated to make them more energy efficient.

Should I Install Cavity Wall Insulation?

Now that you know the importance of cavity wall insulation, there’s no doubt that you should install it, assuming you have wall cavities of course. However, you might already have insulation, though the condition of the insulating material may be questionable.

If your property has been built in the past decade or two, it most certainly has cavity wall insulation already. Unless your home has suffered an insulation problem, such as moisture penetration or mould, the insulation can last many decades.

Yet, your house might not even have cavity walls. Most houses built after the 1920s have cavity walls, while older homes will have solid walls or single skin brick walls, and therefore no cavity to fill. Internal solid wall insulation might therefore be an option.

A simple way to tell whether you have cavity walls is through the brick pattern on the outer wall of your home. Cavity walls usually have a regular pattern. If your home is made of irregular bricks or stone, you have solid walls.

If the bricks aren’t visible, you can also measure the thickness of the walls. A narrow wall will likely be a solid wall, while walls measuring more than 300mm are most likely to be cavity walls.

Cavity Wall Insulation Options

If you’re planning on insulating your home, understanding the different types of insulation is crucial. Each insulation type has its advantages and drawbacks, so it’s best to pick the insulation that’s specific to your needs or requirements.

In some instances where full-fill insulation creating an unbroken thermal barrier is not possible, partial fill insulation may be more appropriate. A partial fill cavity wall leaves a gap between the insulation — and the wall and may not be as thermally efficient — but is typically used as a cost-effective solution when retrofitting existing properties and previously unfilled cavity walls.

Typically, foam is the best cavity wall insulation, as it fits most people’s criteria. That said, some might prefer other cavity wall insulation types, especially if they are more environmentally conscious.

Here’s some detailed information about different types of cavity wall insulation product:

Polyurethane Foam

Polyurethane foam is the most popular insulation material for a reason. It’s the first-choice insulation for new constructions as well as retrospective insulation.

That’s because there are two types of foam cavity wall insulation: boards and spray. Rigid insulation boards are pretty straightforward when added to a new build development. They adhere to the wall through a bonding agent, which means that they are not suitable for retrospective installation as you would need to tear down the wall to install the foam panels.

On the other hand, spray foam can be injected into the cavity through small holes made in the wall. The existing insulation, if it has failed, can be removed through these holes before the new material is applied. After spraying the wet foam into the wall cavity, it expands and takes the form and size of the cavity. Accordingly, it can insulate small cracks and other hard-to-reach areas.

Foam provides an airtight seal, preventing heat loss, moisture, water, and mould growth.The foam can sometimes shrink over time, pulling away from the walls and affecting their integrity.
You can install polyurethane foam in any wall cavities, pretty much regardless of their size or condition.Spray polyurethane foam completely fills the cavity, meaning a lack of proper ventilation.
Since foam is readily available, it’s an affordable insulation method 
Pros and cons of polyurethane insulation


Fibreglass Batts

As the name suggests, fibreglass batts consist of fine glass fibres compressed together to create large, soft panels. Not only are fibreglass batts readily available, but they’re also relatively easy to install during construction.

That said, you can’t really fill an existing wall cavity with fibreglass without significant work being required on internal walls to gain access. Additionally, you mustn’t shred the cavity batts in order to make them fit as the fibreglass can cause a health risk. Accordingly, you can only really use the large fibreglass panels as they are, which can be somewhat limiting.

Batts have a water-resistant coating, which eliminates moisture and mould damage.While fibreglass batts are poor thermal conductors, they’re the least efficient insulators.
They have the highest fire resistance out of all insulation materials.You can’t easily install fibreglass batts retrospectively.
Fibreglass panels are an excellent choice for DIY projects. 
Pros and cons of fibreglass batts


Polystyrene Beads

Another exceptional insulation material is polystyrene beads. You’re probably familiar with these beads as package protection materials and bean bag fillers. Yet, they’re the ideal cavity wall insulation.

Polystyrene beads, or EPS, are made of a carbon polymer, which is a poor thermal conductor. The polymer undergoes steam processing, filling up with air, and acquiring the classic bead shape we all know. This further helps reduce the conduction of heat.

These beads can easily be installed by drilling holes into the wall and then filling the cavity with the beads and a powerful bonding agent.

Polystyrene beads are incredibly simple to install without tearing down the walls.Installing polystyrene beads requires experienced hands and exceptional bonding agents.
The beads are lightweight, so they cause no structural damageThe cavity must be spacious and clean for correct installation.
They absorb absolutely no moisture, keeping mould and water damage at bay. 
Pros and cons of polystyrene bead insulation


Mineral Wool

Mineral wool insulation, also known as rock or slag wool, is the first material of insulation ever used. The insulation isn’t actually wool, though, consisting instead of a mixture of stone and slag.

At exceptionally high temperatures, the stone is heated until it becomes molten. Then, the molten stone cools while spinning to form long fibres. Lastly, the fibres are stacked into soft, dense sheets, hence the name!

The material is inflammable, so it protects your home while insulating it.Mineral wool attracts moisture, so it’s prone to dampness and damage.
Mineral wool acts as an acoustic dampener, making it one of the most versatile insulation materials.The insulation material can be a health hazard.
It’s pretty straightforward to install. 
Pros and cons of mineral wool insulation



While perlite isn’t as popular as other insulation materials, it’s an all-natural product that can fill and insulate established cavity walls. This insulator is lightweight, durable, and inexpensive.

The best part is that perlite is stress-free. You won’t have to worry about the material decaying, damaging the integrity of your home or corroding piping and electrical components. That’s because perlite is an inert, environmentally friendly insulation solution.

Perlite can flow through the cavity, filling it completely.Perlite isn’t as potent an insulator as rock wool or polyurethane fo
It’s resistant to moisture, vermin, fire, and decay.Installing this material requires experience.
This insulation material poses absolutely no health risks 
Pros and cons of Perlite insulation

What Is the Best Cavity Wall Insulation?

Choosing the best cavity wall insulation depends on many factors. For starters, if you’re not in the process of constructing your home, and you don’t want to demolish any walls, fibreglass batts would be out of the question!

In contrast, you have more freedom if you haven’t built your house yet, as you can go for large boards or injectable material. Either way, foam would be an excellent cavity wall insulation option, as it’s cheap, easy to install, and has different forms.

Still, you should take the following into account when choosing your cavity wall insulation:

Efficiency of insulation material

The main purpose of an insulator is to conserve energy and limit heat loss. Naturally, a material that doesn’t do the job properly wouldn’t be considered the best option.

For example, while fibreglass might be a cheap, readily available option, it doesn’t do a good job of trapping heat. Alternatively, foam boards might need professional installation, but they’re ideal insulators.

Wall Cavity Size

Some insulation just isn’t suitable in small or unclean spaces. If your cavity walls are already established, you won’t realistically be able to use numerous materials such as fibreglass, foam boards, and mineral wool.

Fortunately, you can drill holes into your walls and pump the material into the cavity. Though this option is more complicated than if you were to insulate the walls during construction, it’s practical and relatively cheap.

Moisture Resistance

Wall cavities can quickly attract moisture, which in turn attracts mould, and can damage your house’s structure. Accordingly, you don’t want your chosen insulation to act as a catalyst for mould growth.

Not only can this be a health and safety hazard, but dealing with moisture and mould damage can be pretty pricey. For this reason, you should stay away from mineral wool. Instead, opt for foam, perlite, or fibreglass.

Ease of Installation

Cavity wall insulation is typically an easy process, usually taking only a few hours. However, if your walls aren’t easily accessible, or if the materials require special handling, it might take longer.

Accordingly, if you want the insulation to be hassle-free, it’s best to pick an easy, reliable insulator. In this case, you should stay away from polystyrene beads and perlite.

Environmental and Health Safety

Cavity wall insulation is one of the best ways to conserve energy, thus becoming more environmentally green. Additionally, you don’t want to save the environment and harm yourself in the process!

Unfortunately, some types of insulation, such as fibreglass and mineral wool, have some small health or environmental risks, so you may prefer all-natural perlite.


While you can get insulation grants and low-interest loans to help fund your insulation project, you’ll still may need to cover a part of the cost yourself.

Cavity wall insulation, while more expensive than a loft insulation project, is an investment that will more than pay off in the long-term. The insulation materials also last decades, so it’s likely you won’t need to pay for upkeep or re-installation.

According to the Energy Saving Trust, a typical cavity wall insulation installation in a semi-detached gas-heated property would cost around £2,700, but would generate energy bills savings each year of around £280 (based on fuel prices as of January 2024).


Most insulation will come with its own guarantee, often 20 years or more. The Cavity Insulation Guarantee Agency (CIGA) also provides independent 25 year guarantees for cavity insulation fitted by UK installers registered with them. This sort of guarantee gives you greater peace of mind that the cavity barrier insulation installed is fully up to the task.

Membership of professional bodies such as the National Insulation Association (NIA) and the British Board of Agrément are also indications that your insulation installers are signed up to a professional code of practice and therefore high standards will be maintained.

Best wall cavity insulation conclusions

So, what are the best cavity wall insulation materials? Foam is in most cases the first-choice insulator for plenty of good reasons. This material is a good water repellent insulating barrier and exceptional at trapping heat. It’s also relatively straightforward for the professional to install.

The best part about installing cavity wall insulation foam is that it can be installed retrospectively in existing cavity walls or while a property is being constructed. Simply put, foam is the most versatile insulation option and will make a significant difference to energy bills.

Still, there are other excellent insulation materials that can be used for insulating cavity walls. For example, fibreglass is a popular choice. If you want to completely stay away from synthetic materials, perlite is the insulator for you.

Whichever insulation material you choose for your cavity walls, you will enjoy the benefit of improved thermal performance, energy bill savings and a reduced carbon footprint, making your investment pay off over time. Where no wall cavities are present, internal wall insulation or external insulation may be viable options for your solid walls.

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