A cavity wall is a type of wall structure that is made up of two separate walls with a gap or “cavity” between them. These walls are commonly used in modern construction to enhance the building’s performance in various ways, such as thermal insulation, soundproofing, and moisture resistance. The two walls, known as the leaves of the cavity wall, are typically constructed from different materials. The inner wall is referred to as the internal leaf, while the outer wall is called the external leaf. This type of wall is sometimes also referred to as a hollow wall due to the space between the two layers.
Construction Details of Cavity Walls
Cavity walls are constructed with two parallel walls that are separated by a cavity. The space between the two leaves is typically between 4 to 10 cm wide. The thickness of the internal and external leaves must each be at least 10 mm to ensure the wall’s structural integrity. These walls are connected using metal ties or links that help bond the two leaves together, ensuring they act as a unified structure. These ties are typically made of rust-proof steel to prevent corrosion and ensure longevity.
Wall Structure
The leaves of a cavity wall are generally constructed from different materials. The internal leaf is often made from regular bricks, while the external leaf uses facing bricks to provide a more attractive finish. In some cases, a cavity wall can also feature different types of masonry, depending on the design and budget of the project.
Cavity Size and Leaf Thickness
The cavity between the leaves should range from 4 to 10 cm, depending on the type of wall and the required insulation properties. For proper strength, both the internal and external leaves must have a minimum thickness of 10 mm. The cavity is crucial for allowing air to flow through and for preventing moisture penetration.
Metal Ties
To ensure that the two leaves stay together and work as one continuous wall, metal ties are used to connect them. These wall ties are generally made of steel and are designed to be rust-resistant. The ties are placed in both horizontal and vertical orientations with spacing specifications of a maximum of 900 mm horizontally and 450 mm vertically. The positioning of these ties is carefully planned to avoid moisture from transferring from the outer leaf to the inner leaf.
Construction Process of Cavity Walls
Foundation and Footings
Unlike solid walls, cavity walls generally do not require specialized footings. Instead, they are built on a strong concrete base, which provides the necessary support for the wall. This helps reduce the overall construction cost and simplifies the foundation process.
Masonry Work
The construction of the leaves follows standard masonry techniques. The inner and outer leaves are constructed simultaneously, ensuring that the cavity remains the correct size and that both leaves are aligned properly. The cavity is kept clear of any mortar during construction to prevent blockages that might compromise its effectiveness. Wooden battens are often used to support the construction process and prevent mortar from falling into the cavity.
Filling the Cavity
In some cases, the cavity may be filled with lean concrete to add additional stability. This concrete fill is often sloped towards the top to improve drainage and prevent moisture buildup.
Weep Holes
One of the key features of cavity wall construction is the inclusion of weep holes in the outer leaf. These small openings, spaced about 1 meter apart, allow any moisture that might accumulate in the cavity to escape. This helps prevent the buildup of water, which could lead to dampness or mold issues inside the building.
Damp Proof Course
A damp proof course (DPC) is installed along the cavity wall to further protect against moisture. This is typically placed at the bottom of both leaves and may also be added above windows and doors to ensure that water does not penetrate the structure.
Advantages of Cavity Walls
Cavity walls offer several advantages when compared to traditional solid walls. Below are some of the key benefits:
1. Thermal Insulation
The cavity between the two walls acts as an insulating layer. This air-filled gap significantly reduces heat transfer from the outside to the inside of the building. As a result, cavity walls help maintain a stable indoor temperature, making buildings more energy-efficient and comfortable throughout the year.
2. Cost-Effective
Cavity walls are generally more economical than solid walls. The use of air as an insulating material reduces the need for expensive insulation materials. Additionally, cavity walls are lighter, which can reduce construction and material costs.
3. Moisture Resistance
One of the primary benefits of cavity walls is their ability to prevent moisture from entering the building. The space between the leaves acts as a barrier, keeping rainwater and moisture from reaching the interior of the building. The weep holes also allow any trapped moisture to escape, preventing dampness and water damage inside.
4. Sound Insulation
The cavity also serves as a sound insulator. The air gap helps dampen noise from outside, making cavity walls a popular choice for buildings located in noisy areas or those requiring quiet interiors.
5. Reduced Weight on Foundations
Cavity walls are lighter than solid walls, which means they exert less pressure on the foundation of the building. This can lead to savings in foundation costs and improve the overall structural stability of the building.
6. Prevention of Efflorescence
Efflorescence, the white powdery residue that can form on the surface of bricks due to moisture, is prevented in cavity walls. The cavity prevents water from traveling through the outer leaf and reaching the surface where efflorescence can form, helping to maintain the aesthetic appeal of the building.
Conclusion
Cavity walls are a highly effective and efficient building method that offers numerous advantages over traditional solid walls. From providing better thermal insulation and moisture resistance to offering cost savings and soundproofing, cavity walls play an essential role in modern construction. By understanding the construction process, advantages, and structural details of cavity walls, builders and homeowners can make informed decisions that improve the longevity and performance of their buildings.