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Cavity Wall Insulation Guide

What is Cavity Wall Insulation?

Houses built after 1920 are likely to have a Cavity Wall.

Cavity walls are two external walls generally constructed of a concrete block wall and a brick wall with a small gap.

A Cavity Wall was designed to keep Rainwater and Moisture out and reduce heat lost from the Property.

Insulating a cavity wall increases energy efficiency, reduces energy bills and provides more insulation to the Property.

Further benefits of Cavity Wall Insulation include;

  • Acoustic Values

  • Long Term Value

  • Improved Fire Safety

  • Reduces Carbon Footprint

  • Energy Efficient

  • Keeps Property Cool in Heat

  • Insulates against Heat Loss

Cavity wall insulation installed will provide a more Comfortable Home and Save energy in both Winter and Summer Months.

Installing cavity wall insulation could amount to yearly energy bill savings of £200-£500, meaning you could recoup the installation cost in as little as three years.

Rockwool, provide a Handy U Value Calculator to calculate the thermal performance of your home's walls, floor and roof.

Cavity Walls

Before considering installing wall insulation, you should check the following;

  • Wall Type

You may need an expert to confirm your outside walls are suitable for cavity wall insulation, but to check you have a cavity wall first, take a look at the bricks.

Bricks laid lengthways with an even pattern are a good indication you have a cavity wall, whilst half-length bricks may indicate a solid wall.

Your Property is likely to have Cavity Walls if built post-1930.

  • Check your Walls

Check for Damage or any signs of Damp, such as Damp Patches.

This may impact cavity insulation and should be assessed by an expert

  • Location

If you are located in an area with High Moisture from Wind, Rain and Sea, insulation could be impacted over time and may not be suitable.

Types of Insulation Material

The type of Cavity Wall Insulation you use depends on your circumstances.

You may be planning to retrofit cavity wall insulation or build an extension to your Property.

  • Cavity Batts 

A popular option for new installation and can be used on Internal Walls, Outer walls, Floors and Roof Spaces.

Also known as Insulation Slabs, they tend to come pre-packed and can be easily installed, providing Thermal Insulation and Acoustic benefits.

  • Blown Insulating materials

It would be best if you used a Registered Installer for this process, who will drill holes into your external Wall and blow Mineral Wool or Polystyrene beads into the cavity.

  • Foam Injection

Polyurethane foam will form a Thick Water Repellent Barrier and is often used in High Moisture or Flood Risk Areas.


If you plan on a retrofit, you will need advice, and the installation will likely need experts with the equipment and materials.

The National Insulation Association can offer advice and access to Registered Installers for all Insulation Needs.

There are many helpful Installation Guides for Insulation Batts, including this helpful Video from Knauf.



As Climate Change and the Energy Crisis become more entrenched in our Lives, ensuring our Homes remain comfortable all year round whilst reducing Climate Impact and saving us Money is now a Priority.

Properly insulating cavity walls and loft insulation will help with these long-term challenges.

Feel free to contact our Technical Team with your Cavity Wall Insulation Questions. 



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