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 best cavity wall insulation material for your situation. Let’s dive in!

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.

Yet, why is cavity wall insulation so important?

The answer is simple: to preserve heat. 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. However, you might already have insulation. Moreover, not all homes have cavity walls to begin with!

If your house has been built in the past decade or two, it most certainly has insulation already. Unless your home has suffered an insulation problem, such as moisture penetration or mould, the insulation can last a lifetime.

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, and therefore no cavity to fill.

A simple way to tell whether you have cavity walls is through the brick pattern on the outside 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. Narrow walls are solid, while walls measuring more than 300mm are cavity walls.

Types of Cavity Wall Insulation

If you’re planning on insulating your home, understanding the different types of insulation material is crucial. Each insulation material 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 foam boards are pretty straightforward. They adhere to the wall through a bonding agent, which means you need to tear down the wall to install the foam panels.

On the other hand, spray foam can be injected into the cavity through holes. After spraying the wet foam, 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.

That said, you can’t fill an existing wall cavity with fibreglass. The reason is that you mustn’t shred the fibre panels. If the fibreglass comes in contact with your skin or airways, it might cause irritation or worse. Accordingly, you can only 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 the best cavity wall insulation, 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:


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 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 materials just don’t work in small or unclean spaces. If your cavity walls are already established, you won’t 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 damages your house’s structure. Accordingly, you don’t want your insulation material to act as a catalyst for mould growth.

Not only can this be a health and safety hazard, but fixing 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 insulation materials, 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 need to cover a part of the cost.

Most cavity wall insulation is pretty cheap. The insulation materials also last decades, so it’s likely you won’t need to pay for upkeep or re-installation.

Still, since perlite is a natural, less popular option, it can be on the expensive side in comparison to some other cavity insulation materials. On the other hand, polyurethane foam is the cheapest.


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 you have installed is full up to the task.

In Conclusion

So, what is the best cavity wall insulation? Foam is in most cases the first-choice insulator for plenty of good reasons. This material is exceptional at trapping heat and keeping moisture out. It’s also simple to install and inexpensive.

The best part about foam is that you can either install it retrospectively or while constructing your home. Simply put, foam is the most versatile insulation option.

Still, there are other excellent materials you can use. For example, fibreglass is a popular choice as it’s inflammable. 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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