Widely used in domestic construction as an external wall to prevent moisture from entering the building; at any point where the cavity has to be closed an impervious membrane must be inserted. The outer leaf (112mm) is usually face-work in Stretcher Bond, but not exclusively, while the inner leaf (100mm to 125mm) is constructed of light weight blocks with a 50mm cavity. Built as two leaves and tied together (see Wall Ties) to form a single structural unit. To meet thermal insulation standards the cavity is filled with 50mm \insulation bats"or"