The correct answer is: All the above.
A cavity wall is a type of wall that consists of two layers of masonry with an air space between them. The air space provides insulation, both against heat and sound. It also helps to prevent dampness by creating a barrier between the inner and outer walls.
Heat insulation is important in both hot and cold climates. In hot climates, it helps to keep the building cool by preventing heat from entering the building. In cold climates, it helps to keep the building warm by preventing heat from escaping.
Sound insulation is important in both residential and commercial buildings. In residential buildings, it helps to reduce noise from neighbors and the outside world. In commercial buildings, it helps to reduce noise from machinery and other equipment.
Prevention of dampness is important in all buildings. Dampness can cause mold and mildew, which can damage the building and make it unhealthy to live in. The air space in a cavity wall helps to prevent dampness by creating a barrier between the inner and outer walls.
In conclusion, a cavity wall is generally provided for heat insulation, sound insulation, and prevention of dampness.