Cavity walls consist of two layers separated by a hollow space which is called the cavity. The two layers are commonly masonry such as brick or concrete block. The cavity is itself an air insulation layer, but is nowadays normally filled with appropriate insulation materials.
 
Cavity wall
Cavity wall

Depending on the construction type, airtightness is achieved by the inner layer. In general the airtightness and moisture management is provided through the final internal rendering, but the correct positioning of the airtight layers depends on the functional aspects of the inner cavity layer.
 

Overview of solutions for cavity walls

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