Cavity Wall Insulation

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Cavity wall insulation is a smart and efficient way to reduce energy consumption and keep your home comfortable year-round. Most homes built after the 1920s have walls with two layers, creating a gap or "cavity" between them. Cavity wall insulation fills this space with a material that slows the movement of heat, helping to keep the warmth in during the winter and out during the summer. We stock a range of 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?

Cavity wall insulation involves filling the gap between the two layers of a wall that are separated by a hollow space or "cavity." Typically, this is done in homes built with two layers of brick or block walls to prevent heat from escaping and cold air from entering, thus improving energy efficiency. 

The materials commonly used for cavity wall insulation include mineral wool, foam, and polystyrene beads. Each has its properties and suitability depending on the specific requirements of the building and local climate conditions. 

Besides the obvious benefit of energy saving, cavity wall insulation can also help reduce condensation inside the house if it's installed correctly. 

However, it's crucial to ensure that the home is suitable for this type of insulation. Factors like wall condition, exposure to driving rain, and existing damp issues need to be considered, as improperly installed or inappropriate insulation can lead to problems like damp or mould growth.

How Does Cavity Wall Insulation Work? 

Cavity wall insulation works by creating a barrier between the inside and outside of your property, which helps to keep heat in and cold out. 

This not only helps in reducing heating costs but also minimises the carbon footprint of the home by lowering energy consumption. 

The Energy Saving Trust notes that in an uninsulated house, walls account for about 35% of heat loss. Given that walls are the source of roughly one-third of heat escape in such buildings, installing cavity wall insulation is a clear choice for anyone looking to significantly reduce heat loss and lower energy costs.


Cavity Wall Insulation Benefits 

  • Enhanced energy efficiency
  • Reduced heating and cooling costs
  • Improved indoor comfort
  • Minimised heat loss
  • Lower carbon emissions
  • Noise reduction
  • Prevention of dampness and condensation
  • Increased property value
  • Eco-friendly home improvement


Cavity Wall Insulation: Partial Fill vs. Full Fill

Partial-fill cavity insulation involves leaving a 25mm or 50mm gap between the insulation material and the external leaf of masonry walls, reducing the risk of moisture damage to internal structures and is often required by local authorities for new builds. 

In contrast, full-fill cavity insulation offers a quicker and easier installation process by fitting snugly between masonry walls without the need for securing boards or slabs, effectively reducing heat loss and preventing problems related to mortar debris in the cavity space. However, it's important to note that not all buildings are suitable for cavity insulation depending on local building codes.

Types of Cavity Wall Insulation Material

Understanding the different types of cavity wall insulation is crucial for enhancing a building's energy efficiency and comfort. Each material and insulation approach offers unique benefits and considerations. 


Mineral Wool Cavity Insulation

Made from natural rock or slag, mineral wool insulation is fibrous, allowing it to trap air and provide effective thermal and acoustic insulation.

  • Benefits: It is highly fire-resistant and does not degrade over time, ensuring long-lasting performance. Additionally, it is resistant to moisture and can help prevent mould growth.
  • Considerations: The density of mineral wool can make it heavier than synthetic insulations, requiring careful installation. It’s also important to ensure it's packed well to avoid settling or gaps.


Polyisocyanurate (PIR) Board Insulation 

PIR boards are rigid foam panels that offer excellent thermal insulation. They are composed of a polyisocyanurate polymer and are often faced with aluminium foil on both sides.

  • Benefits: PIR boards have a higher insulation value per inch than many other forms, making them ideal for achieving significant energy savings. The foil facing provides a moisture and vapour barrier, enhancing their durability.
  • Considerations: While effective, PIR can be more expensive than other insulation types. Installation must be precise to avoid thermal bridging and ensure airtightness.


Phenolic Foam Boards

Phenolic boards are made from phenolic resin, creating a rigid panel with fine, closed cells.

  • Benefits: It offers superior insulation properties and excellent fire resistance, performing well in both regards compared to many other insulation materials. Phenolic foam also has a low thermal conductivity, making it highly efficient.
  • Considerations: Phenolic foam tends to be among the more expensive insulation options and requires professional installation to ensure optimal performance.

Frequently Asked Cavity Wall Insulation Questions

Is Your Home Suitable for Cavity Wall Insulation?

Not all homes are suitable for cavity wall insulation. It's essential to determine if your property meets the necessary criteria. Generally, cavity wall insulation is suitable for:

  • Homes with unfilled cavity walls.
  • Homes with easily accessible walls where insulation can be installed.
  • Properties with brick or stone walls that have cavities.


How Do I Know If I Have Cavity Walls? 

A cavity wall is made up of two slender walls that are connected by wall ties, leaving a space in between them.

To find out if you have cavity walls, inspect a window or door along an external wall: If the brick wall measures over 260mm in thickness, it likely features a cavity. A narrower wall is more likely to be solid.

If your house is constructed with stone walls, it's probable that they are solid, without any cavities that can be insulated. In such instances, you may want to go down the route of external insulation.

We stock a wide range of External Wall Insulation (solid wall insulation) solutions for projects that are not suitable for cavity wall insulation. 


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.


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