February 28, 2022 Pallika Sood

External Cavity Walls

Written by Harry Copley


In this article, we will be exploring what external cavity walls are and the difference between the materials they can be built from. The External walls of your home serve two main functions: structural and environmental protection. External walls are structurally vital, as they provide the main support for all floors and roofs within your building, therefore they will need to be made of a material that has high compressive strength and is durable. External walls will also need to provide environmental protection to the inside of a home. This means keeping out the cold, the wind, and the rain. For this, any external wall construction must be durable, waterproof and resistant to weather as well as be a good thermal insulator. Many people today also want their external walls to have a soundproofing quality to reduce the noise from the outside.

What is a cavity wall?

The wall construction of choice in the UK for the past 150+ years has been masonry, better known as bricks and mortar; however, the way masonry walls are constructed has developed throughout the years. Houses built before the 1920s typically have external walls made from solid brickwork, usually, two bricks thick, which is approximately 215mm. This type of wall construction will provide all the structural strength and support needed for the building as well as protect from the environment, but not much in the way of thermal insulation and noise reduction. The most significant change in the design of external walls after the 1920s was the implementation of cavity walls. A cavity wall construction is formed from two leaves of masonry, 1 brick which is 100mm thick, separated by an empty void or cavity.

You can work out whether your house has a cavity or solid brickwork external walls by simply looking at the pattern of the bricks. If the wall is made of bricks all the same length, then you have a cavity wall and if the bricks change between long and short lengths, you have a solid masonry wall.

Since the 1960s most houses are built to have external cavity walls with a brickwork outer leaf and a blockwork inner leaf. The modern cavity wall construction used today is a 300mm thick cavity wall made of an external leaf of brickwork and an internal leaf of blockwork with a 100mm thick cavity between them filled with insulation. The two leaves are tied together using steel wall ties to make a wall that is strong and stable with great thermal insulating properties that are aesthetically pleasing to look at from the outside and is durable and resistant to weathering. If you are having your house extended most architects and structural engineers will suggest using this cavity wall construction.

Are you currently having your house extended? Do you need a structural engineering to review your plans? Send your architectural drawings to info@blueengineering.co.uk. 

The difference between brickwork and blockwork

Brickwork refers to any wall made from traditional clay. The bricks are formed out of clay that is then moulded and fired in a kiln. Bricks are highly durable, offering long term performance with very low maintenance required. Bricks also have a high thermal mass allowing them to absorb a lot of heat energy making them a good source of thermal insulation. 

If removed correctly during reconstruction, they can be recycled and reused in a renovation. On some jobs, it is possible to utilise the bricks removed, when demolition takes place, into the new constructions. Bricks can be made in a wide variety of colours and textured finishes, such as black, grey, white, red, orange, blue and yellow. Please note that the colour of the brickwork depends on the clay used to make the brick, for example, bricks made from London clay will typically be yellow. The cost and availability of bricks may be dependent on the location of your house.


Blockwork walls are made from concrete or cement blocks. They are larger than traditional clay bricks, approximately 3 bricks high and 2 bricks long. Blocks may be dense, solid blocks or lightweight, hollow blocks. Lightweight blocks have a hollow core to make them easier to work with as well as improve the block’s thermal insulation properties these blocks are typically used to construct the internal leaf of cavity walls as well as internal walls. Dense blocks are typically solid and are used for more structural purposes such as load-bearing walls and foundations due to their greater strength. 

Dense blocks, however, have poorer thermal insulation properties. Blocks can be made in a wide variety of ways, one thing that will always be the same is the finish and look of a blockwork wall. Blockwork walls will always have finish placed on them such as plastering, painting, tiling or rendering to name a few options. They are cheaper, easier to build and easier to insulate. 

Are you currently having your house extended and aren’t sure what type of wall construction to use? Send your plans to info@blueengineering.co.uk for advice on your best options or call us on 020 7247 3811

In Summary

To Summarise, the external walls of your house are there for two purposes: structural support and environmental protection. If your house was built after the 1920s your external walls will be cavity wall constructions and more modern houses will typically use a combination of brickwork outer leaf and blockwork inner leaf. There are many benefits to using brickwork and blockwork, but the main differences are that brickwork is cheaper and easier to work with, while brickwork is more aesthetically pleasing and does not require any finishes for external walls.

Are you currently having your house renovated and are looking for a structural engineer? Send your architect’s drawings to info@blueengineering.co.uk for a quote.