Cavity Walls Explained
Cavity walls were first introduced in the early 1900s and were initially used in coastal areas, because of the high exposure to wind driven rain.
A solid wall allowed wind driven could totally soak the wall through, leading to the interior face brick (the inner wall) becoming wet and damp. By building houses with two leafs with a gap (cavity)between them, only the outer leaf would get soaked. Any damp moisture that came through the brick work would either evaporate, or run down the outer leaf to the ground.
With changes in building regulations, the use of cavity wall construction became more wide spread . The two leafs of a cavity wall are held together using a series of metal wall ties. However, if the cavity is bridged, either through building debris or sand, then the moisture can travel across the bridge and can affect the inner wall resulting in damp. CWI can act as such a bridge.