Cavity Wall Insulation

Cavity wall insulation prevents heat from escaping through walls, which can account for a significant amount of energy loss in a building. The proof is in the numbers; homeowners lose up to 33% of heat through external walls. Installing cavity wall insulation drastically reduces heating bills - a welcome benefit amid the current energy crisis. We sell boards and slabs (both partial fill and full fill) from industry-leading manufacturers like Celotex, Knauf Insulation, and EcoTherm

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What Is Cavity Wall Insulation?


Unfilled cavity walls can pose a significant threat to the energy efficiency of a project. 

Cavity wall insulation (CWI) is the process of filling the gaps (cavities) between external walls with an insulating material in order to combat heat loss. 

This type of insulation is commonly used to reduce energy bills and combat fuel poverty with some products achieving thermal conductivity values as low as 0.018 W/mK. In the current climate of high energy prices, CWI can be a great way to save money on your home heating costs.

How Does Cavity Wall Insulation Work? 

CWI works by creating a barrier between the inside and outside of your property, which helps to keep heat in and cold out. Cavity wall insulation is designed to fill the space between two masonry cavity walls. They can even be friction fitted between joists and placed on flat surfaces that need to be filled. 

There are lots of different types of cavity insulation on the market from glass mineral slabs, and phenolic boards to PIR boards. These are available in both partial fill and full fill. 

Let's take an in-depth look at both types below. 

Partial Fill Cavity Insulation

Partial-fill insulation leaves a 25mm or 50mm gap between the insulation and the external leaf. Even if moisture makes it through to the outer leaf, there is very little chance that it will bridge through to the internal structure. Local authorities tend to require new build properties to have partial fill insulation inside masonry walls.

Full Fill Cavity Insulation

Full-fill cavity insulation is quicker and easier to install. You do not have to spend time securing the boards or slabs in place. Full-fill cavity batts and boards are designed to fit snugly between the masonry walls, which means that you can simply place them in the space.

Filling the entire cavity reduces the likelihood of future problems and better protects your space against costly heat loss. What's more, you won't have to worry about leaving ‘snots’ of mortar or debris in the space between the masonry walls. 

Please note that some buildings may not be suitable for cavity insulation if their building code does not allow it.

Types of Cavity Wall Insulation - Slabs & Boards

There are two main types of cavity wall insulation that can be installed between solid walls.

Cavity Wall Insulation Slabs
Cavity wall insulation slabs are most commonly made from glass mineral wool. These materials are usually made from at least a third of recycled materials. Unlike rolls of insulation, slabs are flat rigid rectangles that are far easier to install. These are otherwise known as cavity batts.

Whilst slabs are primarily manufactured to reduce heat loss, full-fill solutions like Knauf DriTherm fully fill the cavity whilst boasting water repellent qualities and an A1 anti-combustible rating on the Euroclass scale. They are even BBA certified for use in exposure zones.

Cavity Wall Insulation Boards
Cavity wall insulation boards are made from foam that is sandwiched between two aluminium facings. Because of their foam composition, cavity wall insulation boards are commonly referred to as insulation foam boards or rigid foam.

PIR foam is one of the best thermal insulators available on the market. PIR has a thermal conductivity value of just 0.022 W/mK.

Celotex CW4000
products are the best cavity wall insulation for people who want all the benefits of PIR. They are available in thicknesses ranging from 50mm to 100mm and have low emissivity foil facings. 

The Celotex CW4000 is a range of partial fill cavity insulation, which means that there needs to be an air gap between the insulation and the external leaf.

An alternative to PIR is polyurethane foam and polystyrene beads cavity wall insulation material.

Frequently Asked Cavity Wall Insulation Questions

Does Cavity Wall Insulation Reduce Noise?

Wall cavity insulation is not specifically designed to reduce noise pollution and heating bills. The cavity wall slabs and boards that we stock are designed to reduce the transmission of heat. Despite this, you might still experience an improvement in acoustic performance as a result of cavity insulation.

Sound waves travel through the air from one material to another. In cavity walls, there is a gap between the inner and outer leaves of both external and party walls. By filling this gap, you create a barrier that the sound waves have to move through. These waves will dissipate, meaning less sound will reach the material that is on the other side of the insulation.

If you are looking for insulation that reduces sound transmission, browse through our comprehensive range of acoustic insulation rolls.

Is Cavity Wall Insulation Fire Resistant?

The fire resistance of insulation materials is measured on the Euroclass scale. This scale was created to regulate the classification of insulation materials in Europe. The scale starts at A1 (non-combustible) and ends at F (combustible).

Different types of cavity wall insulation have different Euroclass ratings.

●     Celotex CW4000 - rated E, partially combustible
●     EcoTherm Eco-Cavity - rated F, combustible
●     Kingspan KoolTherm K108 - rated F, combustible
●     Knauf DriTherm - rated A1, non-combustible

An F rating on the Euroclass scale does not mean that you cannot use these products. All these slabs and boards still provide premium protection against heat transmission. 

If you are concerned about the fire rating of these products, you can simply use a build-up of materials that have better heat protection.

For optimal fire protection, we recommend using a cavity barrier in concealed cavities. Cavity barriers block the passage of fire between cavities, containing the spread of fire to a confined area.

Is Cavity Wall Insulation Safe?

Cavity insulation boards and cavity insulation slabs are safe to use. It is a common misconception that some insulation materials, in particular fibreglass, are detrimental to health. 

Many people think that cutting fibreglass insulation can release dangerous particles into the air. Fibreglass cavity wall insulation is only dangerous when it develops mildew and the spores become airborne.

As long as you take the proper precautions when installing (adequate ventilation and PPE), you do not have to worry about dangerous airborne bacteria. Always consult with a registered installer before installing cavity wall insulation. Any reputable installer will be registered with the Cavity Insulation Guarantee Agency (CIGA) and carry out the installation in line with building regulations.

As an alternative to slabs and boards, a cavity wall insulation installer will be able to drill holes in the outside walls and inject insulation. Whilst this process is much more costly, filling cavity walls is not a DIY job. 

How Imperative Is Cavity Wall Insulation in Reducing Heat Loss? 

Cavity wall insulation is as essential as roof insulation when improving the thermal envelope of a building.

Seeing as walls are responsible for approximately a third of heat loss in uninsulated buildings, getting cavity wall insulation installed is a no-brainer for those wanting to radically decrease heat loss and energy bills. 

Among heat loss reduction, there are many other reasons to install cavity wall insulation. Cavity wall insulation is one of the cheapest energy-saving measures you can install. What's more energy saving results in a lower carbon footprint.