The Truth About Cavity Walls

What is cavity wall construction? Cavity walls consist of two separate walls, or “skins,” of masonry with a gap, known as a cavity, in between them. This design was originally developed to stop moisture from getting to the inside of a building. While this system seems strong, older or poorly maintained cavity walls can have issues with moisture penetration and structural integrity over time. These problems can eventually reduce or even negate the initial benefits of this type of construction.

How often is cavity wall construction used?

It’s difficult to see why most builders use this system?

Cavity wall construction is currently used in about 75% of all the housing being built in the UK.

There is little doubt that the major drawback of cavity wall construction is the time it takes and the high risk of weather delay when the bricklayer cost per day is taken into consideration.

Once a builder has used ICF Construction, they might explore the different UK ICF systems as there are significant differences between them but they rarely, if ever, return to cavity wall construction.

Origin: Stone age

Traditional masonry is the oldest building system that developed from the simple process of men putting one stone on top of another until they had built a shelter. Stones evolved into clay blocks hardened in a fire that were called bricks.

Clay Brick Cavity Walls

Bricks were originally built in solid walls two bricks thick but these let moisture travel through the wall and the solid wall was divided into two walls with a cavity between them but the two had to be connected with metal ties for strength.

CAvity Wall Insulation

However the walls allowed valuable heat to be lost from the interior and so the cavity between the two was filled with slabs of dense mineral wool fiber that became known as cavity wall insulation. In June 2022, the width of the cavity had to be increased to 120mm to provide enough insulation.

Cavity Wall Insulation Problems

The width of the cavity has increased from 50mm to at least 120mm. In this way, a simple method of construction has evolved into one of high complexity that is very difficult and time consuming to build and needs many local bricklayers. The width of the wall has also reduced the interior floor space that is provided in a typical detached house by 8 – 10%.

Cavity Wall Problems

Design Issues

If the cavity wall insulation is not suitable or was incorrectly installed and not properly sealed, moisture can easily penetrate the wall. This leads to issues such as dampness, mould growth, and potential damage to the structure over time.

Another common problem with cavity wall construction is detecting and repairing issues within the cavity. It can be challenging without specialised equipment and can make maintenance and repairs more complicated and costly.

Building a cavity wall requires careful construction techniques to ensure both leaves are correctly aligned, properly tied together, and adequately insulated. If not done correctly, it can lead to structural issues.

In the event of a fire, the cavity can allow flames to spread quickly between floors or sections of a building. Fire-resistant cavity barriers are essential to mitigate this risk. The cavity wall insulation itself can pose a fire risk if it has been installed incorrectly with exposed gaps or tightly compacted areas. In addition, the cavity insulation material itself can contain combustible components.

Comfort

Most people tend to think that a house that is seemingly built of solid bricks will enjoy lots of thermal mass just like those stone farmhouses in France, Spain and Italy that never get too hot in the summer. Nothing could be further from the truth!

Modern homes built with cavity wall construction where there is an outer skin of bricks or rendered dense concrete, a 120 mm cavity filled with rockwool insulation and an inner loadbearing skin of lightweight blocks that provide an essential part of the wall insulation. As a result cavity wall construction provides very little thermal mass. As a result blocks of flats, terraced houses and even semi-detached houses get uncomfortably hot in the summer. Read more about the effects of thermal mass on modern home comfort levels here.

Insulation materials within the cavity can settle over time which reduces their effectiveness. Additionally, if insulation becomes wet due to moisture penetration it can lose its insulating properties. Equally, if there is too much ventilation the cavity wall insulation can degrade and increase the risk of fire spreading.

The new rules in the June 2022 changes to the Building Regulations require every new dwelling to be tested for air leakage. When a house built with cavity walls fails to meet regulation standards the costs of repair can be very high. Furthermore, if the cavity wall insulation has not been installed correctly air pockets or gaps will facilitate air circulation in the cavity. Air leakage will make the house hotter in the summer and colder during winter which will affect the comfort level of the occupants.

Bricklayers

Lack of local bricklayers: what does this mean for you?

National Housebuilding Council (NHBC) estimates that 33,000 more bricklayers are needed to meet the government’s target of building 300,000 new homes each year.
Home Builders Federation estimates that 2,500 local bricklayers are needed for every 10,000 homes built.
The number of local bricklayers in the UK has been declining, with a 35% drop between 2004 and 2020.
The UK is experiencing an aging workforce, with many builders over 50.
The construction industry lacks training for young people.
The pandemic caused many migrant workers to return home and many Polish workers left the UK after Brexit because of the booming economy in Poland.
Average bricklayer cost per day is between £240-£320.

THE COST

The use of facing bricks has been traditional in the UK for hundreds of years but is becoming rarer and rarer because they are so expensive to make and lay.
The 3 or 4 months that it takes to get the roof on – in good weather – delays the finishing trades and increases the cost of expensive scaffolding!

FAILURE RISK

To meet current Building Regulation requirements for draught-proofing and cold-bridges, masonry construction needs to be of the highest standard.
The very wide cavity and long brick ties now required makes it much more difficult to build a cavity wall. The construction quality relies far too much on the skill and reliability of the bricklayers.

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