Cavity Wall Insulation or External Wall Insulation?

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What is cavity wall insulation?

Cavity wall insulation (CWI) is a type of insulation inserted into the space inside the walls of a property – the cavity. Cavity walls are formed by two walls of brickwork built close together, between which the insulation is injected.

Most homes built after 1920 have cavities, whereas older homes tend to have solid walls. More modern properties are usually constructed with a layer of insulation boarding between the walls, especially if they have been built in the last 20 years.

There are many types of CWI for different property types, including: mineral wool, bead or foam. Some CWI materials may not suitable for every cavity (especially if they are uneven or narrow), therefore a specialist survey is required to determine suitability.

Insulating your cavity walls can save up to £150 per year on your heating bill if you live in an average semi-detached house.*

*Figures taken from Energy Saving Trust

What is external wall insulation?

External wall insulation (or EWI) by contrast, is insulation fitted to the outside of a home, where cavity walls are not present. These tend to be properties of solid wall construction, or system built homes with steal or timber frames.

EWI will change the visual appearance of a property and many residents choose to install it to improve the aesthetics of their home. Whereas homes suitable for CWI will look pretty much the same once the insulation has been installed (other than drill marks that will be covered with colour matching mortar).

Installing EWI can save up to £255 pounds per year on your heating bill if you live in an average semi-detached house. Residents in larger homes could save up to £425 annually.**

**Figures taken from Energy Saving Trust

Learn more about EWI in our Simple Guide to External Wall Insulation.

How do I know which one is right for my property?

To choose the right insulation system, you need to establish whether your home has cavity or solid walls. If you do have a cavity then CWI is what you require. If you don’t have cavity walls then you’ll need EWI.

How to tell the difference between a brick cavity wall and a brick solid wall:

The brick pattern normally tells you.

Cavity walls have a regular pattern, with all bricks being placed with the long side of the brick facing forwards. The pattern for solid brick walls will be alternating:

If your property has a cavity you may be able to spot weather it has already been insulated by looking for drill marks. These are typically 1.2 meters apart and will have been resealed to blend in with the existing brick work.

Some brick walls are covered with render or cladding, which means you can’t use visual cues from the brickwork to establish whether you have cavity walls or solid walls.

If you have an Energy Performance Certificate (EPC) for your home you will be able to see what type of construction the walls are. This is typically stated on the second page of the certificate.

Check the Landmark register to see if your home has an EPC

Steel-framed, timber-framed or pre-fabricated concrete walls:

Non-traditional walls such as steel-framed, timber-framed or system built concrete walls can, in most cases, be fitted with EWI.

Identifying these kinds of walls can be difficult and you will normally need a professional to help you establish whether your property is of non-traditional construction, as there are a lot of different types of non-traditional properties each with their own characteristics.

Some professionals use a publication called the BRE Report 469 Non-traditional houses: Identifying non-traditional house in the UK 1918-1975. This publication has a full list of the different types of system built properties used in the UK. You can access a digital version of the report here.

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