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What is a Cavity Wall

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Cavity wall construction is required for any modern masonry built home in the UK. Such walls are double layered consisting of brick and concrete blocks, with a gap between them. In general, most homes built after the year 1920 are cavity (double layered walls) rather than single layer walls. The exterior of the wall is made from bricks, and the interior layer is made from aggregate concrete blocks.

The interior and exterior walls would be tied together using cavity wall ties. The stainless steel ties are built into the wall using mortar during construction, and simply bridge between the internal and external walls at regular intervals. Their purpose is to provide structural stability, helping to counter any bending forces acting on the walls. Below is a diagram of a typical non-insulated cavity wall. The wall ties have not been shown in the diagram, but you can view cavity wall ties on the Amazon store to see what they look like.

Standard UK Brick Size = 215 (length) x 102.5 (width) x 65mm (height)

Standard UK Concrete Block Size = 440 (length) x 100 (width) x 215 (height)

Types of Concrete Block

These concrete blocks are sold in 3 density’s, being lightweight, medium and dense blocks. More density equals higher compressive strength and acoustic insulating properties. The strength of a block is given in Newtons per millimetre square (N/mm2), usually in the 3.6N to 7.3N range. Higher Newton blocks would be denser, stronger and heavier. And offer less thermal properties, and more acoustic properties. The opposite is true for low density blocks.

Lightweight Blocks

Also known as thermalite breezeblocks, these blocks contain lots of tiny air bubbles. This creates a structure that is incredibly lightweight with excellent thermal insulating properties compared to other types of concrete block. This makes them ideal for interior house walls. They are not as strong as the medium and dense concrete blocks though. Buy lightweight 3.6N concrete blocks from the Amazon store

lightweight concrete block

Medium and Dense Concrete Blocks

Offer excellent strength and acoustic properties. Block compressive strengths can go up to 40N, which is incredibly strong. Dense blocks like the ones below are manufactured with dense aggregate, which means they are more durable and robust in building applications where strength is required. Buy dense 7N concrete blocks from the Amazon store

Prevent Damp with Cavity Walls

Cavity walls help prevent damp entering the home. It does this because rain water can only penetrate the outer brick layer, then eventually evaporates within the cavity. This means that the inner brick layer should never come into contact with water, and therefore does not get damp. You will notice in the diagram below that rain water can travel through the porous exterior brick. The air gap (or cavity) between the layers help vent off the moisture from this rain water. This helps to keep your interior walls dry, thus preventing damp. The second image below shows an additional method that is used to control water ingress. All cavity walls should have wall ties built into them during construction. These wall ties are also designed to drip away any water that has penetrated through the exterior brick wall. This controlled dripping helps to prevents the water entering the concrete blocks, or building up on the brick wall.

Diagram showing what is a cavity wall
cavity wall tie drip

Insulated Cavity Walls

Cavity walls provide more thermal insulation over single layered ones due to the extra layer of brick, and the air gap between them. But a lot of heat is still wasted as this brick is inefficient at trapping heat. This is because masonry is a poor thermal insulator, as heat can easily pass through it. Different materials conduct heat more than others. In order to trap heat in your home, you need a material that is a poor conductor of heat such as a mineral fibre.

Below is Knauf cavity wall insulation sold at the Amazon store

The mineral fibre insulation below is 50mm thick. It comes in slab form, making it ideal for cavity wall installation. These slabs are most suitable when constructing new cavity walls where the cavity is easy to access.

cavity wall insulation

Below is a diagram of how insulation would be installed on a new build within the UK.  You will notice that the insulation does not fill the whole width of the air gap (or cavity).  This is to provide ventilation for any moisture that penetrates the exterior brick, preventing it from bridging across to the interior wall.  Any moisture entering the interior wall would eventually lead to damp and possibly mould.   

Insulated cavity wall diagram
Insulated cavity wall moisture ventilation diagram

Thermal Conductivity of Brick and Insulation

Each material has a particular thermal conductivity value, measured in W/m.K. The higher the value, the easier heat can pass through the material, meaning a poorer heat insulator.

The thermal conductivity of brick is 0.73 W/m.K. This is considered a poor heat insulator compared to mineral fibre. Heat will be lost at a high rate if you only rely on bricks to hold in the heat.

The thermal conductivity of mineral fibre insulation is 0.035 W/m.K. Which is a very good heat insulator compared to brick (0.73 W/m.K). This is why mineral fibre is generally used to insulate cavity walls.

The diagrams below show how the heat is transferred through your wall with and without the mineral fibre insulation. The top diagram shows more heat being transferred from the inside to the outside. Whereas the second diagram below shows that most heat bounces off the mineral fibre back inside the house.

Non-insulated house wall heat transfer diagram
Insulated house wall heat transfer diagram

How to Insulate a Cavity Wall

If you already have uninsulated cavity walls, you may be thinking how you go about insulating them. Well, the solution is simpler than you may think. A qualified contractor can fill your cavity with polystyrene beads, polyurethane foam or mineral wool. This is done by drilling holes through your external wool, large enough for a hose to enter. They would then use the hose to blow the insulation into your cavity. However, if your house is new, then the builders may install slabs of insulation during construction. As they would have easy access to the cavity.


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Posted on 25th December 2020 at 8:22 am

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