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Cavity Wall Insulation Pros and Cons

Do you have a cavity wall in your home? If yes, you need to understand the type of insulation material used and determine the best insulation option or even assess if cavity wall insulation is the right choice for your property.

Or are you wondering which insulation materials are optimal for your residence? Or you're keen to assess the quality of the existing insulation in your cavity walls to determine if an upgrade is in order.

Look no further! This guide will discuss a comprehensive overview of various cavity wall insulation pros and cons and their types.

Cavity Wall Insulation Pros and Cons

What Is A Cavity Wall, And Why Do You Need It?

Cavity wall insulation is primarily used to minimise heat loss through the space between walls and to enhance ventilation. Many homeowners favour it not only because it reduces energy consumption but also because it acts as a barrier, preventing rainwater from seeping through the outer wall and entering the home.

Everyone seeks ways to cut expenses and make home maintenance more economical. Insulating your home is one of the most efficient and straightforward methods to achieve this.

The government also supports this initiative, providing incentives to encourage homeowners to insulate their walls and attics. This not only aids individuals but also contributes to the nation's goal of reducing CO2 emissions — a commendable and eco-friendly move.

However, before making any commitments, you must understand what you opt for and how a specific insulation type might impact your home. Moreover, it's crucial to determine if your house is even suitable for cavity insulation retrofitting.

How Is Cavity Wall Insulation Installed?

There are multiple cavity wall insulation materials available in the market. However, the most common ones are polystyrene beads.

These tiny beads are introduced by drilling holes, typically around 22-26 millimetres wide, spaced about one meter apart on the exterior wall.

Special equipment is then used to inject the insulation materials. An adhesive is employed during the injection to ensure the beads remain in place. These beads are favoured over other cavity wall insulation materials for several reasons. Their small size allows easy injection into even the tiniest cavities and distributes evenly within the cavity walls.

General Cavity Wall Insulation Pros And Cons


  • It minimises the heat loss through your home's walls.

  • It can eliminate draughts within your home.

  • It helps in lowering noise transmission via your walls.

  • The installation cost is reasonably low.

  • It preserves the external appearance of your home.

  • The installation process is straightforward and quick.

  • Properly installed insulation can mitigate condensation issues.


  • Certain cavity insulation materials may sag as they age.

  • If water penetrates your cavity insulation, its efficiency in keeping your home insulated will be reduced.

  • Filling the cavity with insulation removes the gap that formerly acted as a moisture barrier for the inner layer of the wall.

  • If not installed correctly, its effectiveness diminishes.

  • The depth of insulation to be used is determined by the width of the cavity, usually 75mm or less.

Which Homes Are Suitable For Cavity Wall Insulation?

If the old cavity has a width of 50 millimetres or more, it's recommended to opt for cavity wall insulation to minimise heat loss. Furthermore, if your home was constructed over 40 years ago, it likely needs to be adequately insulated, thus making insulation installation a wise choice.

Assuming all necessary conditions are met for your home – such as its suitability for cavity wall insulation, adequate ventilation, and installation by certified professionals – cavity wall insulation can be an excellent choice. It can effectively elevate the core temperature of your home, leading to significant savings on your long-term heating bills.

This not only aids in preventing dampness in your home but is also beneficial for the environment. However, being informed about the insulation materials within your cavity walls is crucial. The UK's primary cavity wall insulation materials include mineral wool (rock wool), cavity foam insulation, polystyrene bead insulation, and cavity batts.

Popular Cavity Wall Insulation Materials

The following are the most common cavity insulation materials, along with their pros and cons.

Please note: The efficiency of cavity wall insulation materials is indicated by its λ-value (lambda value). A lower λ-value signifies better insulation because it means reduced thermal conductivity. In simpler terms, the better the insulation's ability to resist heat flow, the less heat will escape through it.

1: Mineral Wool (Rock Wool)

Mineral wool is one of the first cavity wall insulation materials, gaining popularity in households during the 1950s. Typically crafted from igneous rocks, this material undergoes a heating and spinning process to become what we recognise today as mineral wool.

While it remains the insulation material of choice for many residential spaces, it has advantages and drawbacks.


  • Abundant Raw Materials: Derived primarily from igneous rocks, but can also be made from recycled glass and sand.

  • Environmentally Friendly: Due to its recyclable nature and origin from natural and recycled sources.

  • Ease of Installation: Easily fits between inner and outer walls and can be tailored to cavity width.

  • Cost-Effective: Approximately £10-15 per square metre (excluding labour).

  • Effective Insulation: Has a commendable lambda value of around 0.034 W/mk, minimising heat loss and thus reducing heating bills.


  • Requires Clean and Spacious Cavity: Mineral wool cavity walls must be inspected and cleaned before installation, which can be costly.

  • Not Suitable for Small Cavities: Inadequate cavity space can lead to more harm than benefits.

  • Prone to Moisture Issues: Despite claims, it isn't water-resistant, leading to potential damp issues.

  • Clumping Issues: When damp or old, it can clump around brick ties, jeopardising structural integrity.

  • Expensive Repairs: Damages due to mineral wool insulation failure can be costly to mend.

2: Polyurethane Foam

Polyurethane foam (PUR) insulation has been available to homeowners since roughly 1986 and is a common choice in newly constructed homes. Essentially, polyurethane is a type of plastic comprised of organic units, typically carbon-based, which bond to form a stable polymer.


  • Air Barrier: Limits warm air loss from the home, improving energy efficiency.

  • Impressive Insulating Performance: It has a lambda value of 0.030 W/mk, making it one of the best insulators, which aids in reducing energy costs.

  • Cost-Effective: While not the cheapest, at £20-25 per square metre (excluding labour), its superior insulating qualities make it a valuable option.

  • Resistant to Mould: As an inert polymer, it doesn't serve as a food source for mould, even in damp conditions.


  • Potential for Water Damage: Can shrink over time, leading to water infiltration in uninsulated gaps. Hence, proper installation is crucial.

  • Size Reduction: The material may shrink over time, reducing its insulative effectiveness and separating from the wall framing.

  • Compromises Cavity Ventilation: While its expansive nature ensures thorough insulation, it can hinder the cavity's breathing ability, leading to potential dampness issues.

3: Polystyrene Beads

Expanded polystyrene beads, commonly called EPS, are one of the most popular cavity wall insulation materials today, crafted from a carbon polymer and infused into your walls using a bonding substance. Initially, these carbon polymers are about 0.5mm across.

Yet, they undergo a steaming procedure to achieve the familiar shape recognisable today. This process inflates them with air, resulting in a larger bead size, typically between 3-5mm in diameter.


  • Durable and Lightweight: Easily installed due to their light structure and no bulky materials.

  • Bonding Agent Use: Ensures a clean installation with minimal mess.

  • Cost-effective: Affordable at £10-15 per square metre (excluding labour).

  • Water Resistance: Low absorption ability helps prevent dampness.

  • Good Insulation: λ-value of 0.03-0.034 W/mk provides efficient insulation.

  • Recyclable: Can be reprocessed into various products.


  • Requires Clean Cavity: Installation demands a clean space, which might incur cleaning costs.

  • Installation Precision Needed: Incorrect installation, especially with an inadequate bonding agent, can lead to issues like the beads falling out.

  • Potential for Gaps: Beads might pull away, or the bonding agent may degrade, leading to heat loss and potential dampness.

4: Cavity Batts

Cavity batts, utilised for roughly three decades, are commonly found in newly constructed homes, extensions, and commercial buildings. These insulation materials are basically compressed mineral wool pressed into slabs that vary in thickness from 50mm to 150mm and are encased in a moisture-resistant layer.


  • Lightweight & Easy Handling: Reduces risk of damages and makes installation easier.

  • Cost-Effective: One of the more affordable insulation options, depending on thickness.

  • Close Inner Cavity Fit: Allows for proper ventilation and lessens the risk of dampness.

  • Water-Resistant Coating: Ensures no moisture absorption, further minimising damp risks.

  • High Fire Resistance: Holds the top Euroclass Classification (A1).


  • Higher λ-value: At 0.037 W/mk, it's less efficient than other insulation materials.

  • Installation Limitations: Suitable only during the construction phase, not for retrofitting in existing homes.

Which Is The Best Cavity Wall Insulation Material?

If you're in the process of constructing a home, we strongly suggest opting for cavity batts. While they might not offer the top-tier insulation performance seen in some other materials, they perform commendably.

The advantages of using cavity batts, such as ease of installation during construction, cost-effectiveness, structural integrity, and safety, make it a worthwhile choice for new homes.

On the other hand, there might be better options for already-built homes than cavity batts. Instead, we suggest considering polystyrene beads as an insulation solution. While the installation process can be sensitive to errors, EPS emerges as an excellent choice if executed correctly.

It is affordable, ensures clean installation, offers efficient insulation, and is recyclable, making its advantages prominent over its downsides.


Regardless of the insulation type you choose, whether for retrofitting existing cavities or for a new build, they all function similarly by capturing air (or gases). Their performance reduces when wet, so it is crucial to ensure the insulation remains dry.

While there are numerous cavity wall insulation pros and cons, their effectiveness is determined by the specific insulation type. Thus, it's essential to ascertain that your home is compatible before embarking on the insulation journey.

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