Cavity wall extraction refers to the process of removing material from the gap (or cavity) between the two layers of a cavity wall, typically in buildings with external walls consisting of two layers, a brick outer layer, and an inner block or brick layer.

This cavity is often used for insulation or to allow moisture to escape.

In the context of insulation, cavity wall extraction involves removing old, ineffective, or damaged insulation material, such as foam or mineral wool, which can degrade over time or get wet.