Masonry cavity walls can be extremely efficient at preventing water problems. The cavity is the key. Water passing through the outer wythe flows down the inside face of the wythe to flashing which guides the water to weep holes for exit back to the outside. In a properly constructed cavity wall, water penetrating the outer wythe is prevented from migrating to the inner wythe, where it could penetrate the masonry and cause damage to interior wall surfaces. Although cavity walls should prevent water migration from outer wythe to inner wythe, water also can come in contact with the inner wythe in other ways, such as roof leakage or parapet failure. The effects of moisture condensation also should be considered. Construction flaws, such as mortar bridging and incompletely filled mortar joints, also can compromise cavity wall performance. Waterproofing the backup wythe can help prevent these problems. Several methods can be used to waterproof the outer surface of the backup wythe in a masonry cavity wall. PARGING Parging involves plastering the outer surface of the backup wythe with not less than a 3/8-inch thickness of rich cement mortar. To enhance its water resistance, a waterproofing admixture often is added to the mortar during mixing. MEMBRANES Waterproofing membranes often are used in walls subjected to considerable water infiltration. MASTICS OR SEMI-MASTICS Asphaltic mastic or semi-mastic coatings often are fiber- reinforced to resist cracking. Mastics are applied with a trowel, while semi-mastics generally are applied by brush or spray. SILANE AND SILOXANE SEALERS Sealers, such as silanes and siloxanes, generally are applied by low-pressure spraying. These sealers penetrate the masonry surface up to 3/4-inch and chemically react with the masonry to create a waterproof silicone resin within the masonry's pores.