For the average home in the UK, the installation of cavity wall insulation can be one of the best ways to drastically reduce your heating bills and improve the energy efficiency of your house in a practical and cost-effective way.

For any house built with cavity wall, this form of insulation is something you ought to seriously consider. This article looks to answer some of the key queries surrounding cavity wall insulation, helping you to make the best decision for your home.

What is cavity wall insulation?

Cavity walls are a feature of the vast majority of brick-based houses built after the 1920s in the UK. This building technique makes use of two layers of outer walls, leaving a small space between the two. The cavity created is installed to prevent rainwater, frost or snow from penetrating the outermost wall of the house, helping to prevent damp from creeping inside the property. While this has been a popular and successful building approach for nearly a century, an unfilled cavity could be to blame for a third of all the heat lost from an inhabited household, damaging the energy efficiency of your home.

Image alt text: yellow cavity wall insulation in between two brick walls

Typically made from cellulose insulation, glass wool or polyurethane foam, cavity wall insulation is a fibrous material that can be used to fill the gap between the two layers of outer walls. This acts to reduce heat loss by preventing convection, making sure your house stays warmer.

Have I got cavity wall insulation?

It’s not always easy to check if wall insulation has already been fitted in your house, but there are a few things to remember when looking into this.

For older buildings, cavities can be filled by injecting an insulating material through small holes drilled into the outer walls of a house. You can check if this has already been completed by looking for small circles in the mortar between the bricks all the way up the sides of your house. These circles will be spaced roughly a metre apart and are approximately the size of a 50 pence coin. The presence of these filled-in holes indicates that an insulating material has already been blown into your home’s cavity walls.

When it comes to newer houses - generally those built after 1990 - wall cavities have usually been filled as standard, with panels of fiber glass wool or rock wool panels being installed during construction. 

How does cavity wall insulation work?

In order to understand how cavity wall insulation works, first think about how you wrap yourself up before going outside during winter. A jumper, scarf, hat and overcoat all keep you warm by preventing your body heat from escaping easily. Cavity wall insulation works in a similar way.

This fibrous material replaces the air in the gap between the two outer walls of your home, preventing heat from breaking free. Air acts as a natural insulator, so by filling this empty gap in a house’s walls with a wool-like material, your home can trap the air in between the dense fibres of the insulating material and drastically reduce the amount of heat lost through walls.

How much does cavity wall insulation cost?

Naturally, the installation cost of cavity wall insulation will vary depending on the size of your property and the type of walls involved. However, as a rule of thumb, the process should cost around £200 per two-story, outward facing wall. With this in mind, an average detached house with four external walls may cost around £800, while a mid-terrace property with just two outer walls, or one story bungalow, could cost less than £400.