A wall built with two sets of bricks or blocks, with a gap, or cavity between them. Cavity is usually about 50mm in modern construction but can be much thinner. There are some very early examples of cavity walls from the 1860s but they became common place in the area from about 1890. Dependent on the age of the property, there may or may not be insulation between the two leaves. Insulation can be retrospectively installed by drilling holes in the mortar and blowing insulation into the holes. This is only recommended once the condition of the metal ties that tie the two leaves of wall together has been checked by an expert.