What is a Cavity Wall?
A cavity wall, as the name suggests, is a hollow wall with a cavity (or space) between them as two separate walls, known as skins or leaves. The two walls are made to act as one wall to improve insulation in cold countries. The interior and exterior walls are called internal and external skins, respectively. These leaves should be at least 10 cm thick.
The thickness of the two leaves can be designed according to structural requirements. Both walls can be made of the same thickness to act as a non-load-bearing cavity wall. Alternatively, the inner leaf may be thicker than the outer leaf if the cavity wall withstands an applied load.
The two leaves of a hollow wall must be connected by some kind of joint. This can be done using metal pins or spaced connecting bricks. The cavities between the leaves can be empty or filled with heat-insulating and water-repellent materials and have almost zero bearing capacity. The size of the cavity is usually 4 to 10 cm wide.
How to construct a Cavity wall?
The two leaves of the cavity wall are made using conventional masonry processes with a gap of at least 4 cm and the cavity can be left empty or filled with lean concrete or other waterproofing insulation. The top of the cavity fill, a few centimeters above the ground is slightly sloping. Cavity walls do not require supports below them and are instead built on the center of a solid concrete base.
For the outer leaf, weep/drain holes are provided at 1m intervals at the bottom. Cavity wall leaves are built using a variety of masonry and the bricks used for the interior and exterior leaves are plain brick and facing brick respectively. The top of the wall must be stiff and solid to evenly distribute the load on both leaves.
These leaves are usually connected by wall ties or metal pins and are designed to prevent moisture transfer from the outer sheet to the inner sheet. Metal pins are usually made of stainless steel. The maximum horizontal distance between these wall ties must not exceed 900 mm and the maximum vertical distance must be less than 450 mm. These wall pins are available in a variety of shapes and sizes.
As for the type of bond, the stretcher bond is provided for semi-brick leaves, and if the inner leaf is thicker than one brick to carry heavy loads, an English or Flemish bond is provided. When laying bricks for leaves, appropriate precautions must be taken to prevent the cavity from being filled with cement mortar.
Appropriately sized wooden battens are provided on the wall ties to prevent the cement mortar from falling into the cavity. Once the wall is assembled to the height of the new wall tie, the batten is removed with wire or rope to install the new wall tie. A minimum number of vents should also be provided in the cavity for ventilation to avoid excessive humidity.
The two leaves of the cavity should also be created at the same time to avoid any dimensional errors. Also, care must be taken to maintain equal spacing between the two leaves that can be achieved by predetermining the position of the ties on the wall, and a damp proof course (DPC) must be provided separately for the two leaves. The cavity should be filled with fine concrete up to 15 cm below the DPC. For doors and windows, weep holes should be placed above the DPC
Types of Insulation used in Cavity Walls
The most common types of insulation used inside cavity walls are:
01 - Rockwool
Rock wool, also known as mineral wool, is commonly used insulation for cavity walls in homes. This type of insulating wool is made by heating igneous rock and spinning it into a fibrous material. Mineral wool is blown into the walls of the cavity and is filled. It is waterproof and can easily prevent the transfer of moisture through the wall from the outer leaf to the inner leaf.
02 - Polystyrene Bead
EPS or expanded polystyrene(styrofoam) beads are used as fillers for hollow walls. These beads are mixed with glue to hold the beads together and prevent them from spilling when pushed into the cavity. These insulating beads, usually used in cramped hollow walls, are generally preferred over rock wool, especially in stone homes.
03 - Foam Insulation
Cavity foam insulation is usually made from urea-formaldehyde. Urea-formaldehyde can be easily inserted through a very small hole in the cavity wall. These holes can be drilled in cavity walls or masonry. Polyurethane is also used as cavity insulation to provide additional stability to the wall structure if the wall anchors cannot hold the sheet together. Foam insulation is not commonly used.
Purpose of a Cavity Wall
The cavity wall is generally provided for the following purposes:
- Dampness Prevention
The cavity wall keeps the inside of the building dry by preventing the transport of moisture from the outer leaf to the inner leaf. Ventilation in the cavity allows the proper amount of air to flow into the cavity and helps the moisture evaporate, thus blocking the movement of this moisture through the cavity.
- Sound Insulation
The air inside the cavity act as a good sound insulator. The air acts as a sound-absorbing cushion and blocks the sound moving through the cavity and thus keeps the building insulated from sound.
- Thermal Insulation
The air inside the cavity also acts as a thermal insulator for the building.
Cavity walls provide 25% better thermal and sound insulation than solid walls at a lesser cost.
Cavity wall Advantages
- Cavity walls prevent the movement of moisture or rain from the external leaf to the inside of the building.
- Since air is a non-conductor of heat, the air inside the cavity prevents the transfer of heat from the outside inside the building. Therefore, these walls keep the insides of a building warm n winters and cool in summers.
- Air is also a good insulator of sound and therefore the air in the cavity prevents sound from traveling through the wall. Cavity walls are better sound insulators than solid walls.
- Cavity walls fire resistance of the building.
- Cavity walls greatly reduce the risk of condensation of moisture.
- The overall cooling cost of a building can be reduced by using cavity walls.
- Cavity walls also protect the building from efflorescence.