Cavity Walls construction is a modification of cavity walls that were developed to prevent rain penetration through the masonry by providing a drainage cavity in the wall. The cavity was an ideal place for the insulation but filling the cavity with insulation diminished its value as a drain for moisture.

Cavity Walls design calls for insulating the exterior faces of the inner wythe to provide for the structural and insulation components of the wall system. This wall should also meet the requirements for an air barrier in the construction. A clear ventilated space is maintained to the outer wythe which has a number of vent holes to assure there is no wind pressure differential across the wythe to aid penetration of rain.

The exterior wythe provides the exterior finish for the building. Canadian Building Digests 40, 44 and 50 by the Division of Building Research (now the Institute for Research in Construction) at the National Research Council provide design information for this construction.