A cavity wall is an external wall built as two separate leaves of masonry with a gap between them, tied together with wall ties. The cavity prevents rain penetrating from the outer leaf to the inner leaf, and in modern construction it also carries the insulation.
Cavity wall construction is the traditional masonry approach in Ireland. The outer leaf takes the weather, the cavity interrupts the path for moisture, and the inner leaf provides structural support and the internal finish. Wall ties across the cavity keep the two leaves acting together.
The details that make a cavity wall work are the cavity trays, damp proof courses, weep holes and closers at openings. Where these are omitted or bridged by mortar droppings, water can track across to the inner leaf and cause damp.