I have heard people recommending asphalt coatings for the exterior face of the concrete masonry back-up wall in cavity wall construction to act as a vapor-retarding membrane. This is particularly true with an exterior wythe of glazed brick. If the intent is to create a vapor-retarding membrane on the surface of the block, wouldn't it be better to use an impervious sheet membrane, such as rubberized asphalt? The surface of the concrete masonry is typically very rough, and an asphalt coating would typically contain pinholes.
Applying a rubberized asphalt membrane over the surface of the concrete masonry would make a more effective vapor-retarding membrane than would an application of an asphaltic paint. However, before applying any coating, you must ask yourself whether this is the proper place for a vapor-retarding membrane. A vapor-retarding membrane should be on the warm side of the insulation. Therefore, over a large portion of the United States, the heating season dictates that the vapor barrier be on the interior side of the insulation. For insulated cavity walls, this would be the exterior surface of the concrete masonry backup wythe. However, when insulation is not placed in the cavity, it would typically be either within the cells of the masonry backup or on the interior surface of the concrete masonry. In these cases, applying an asphaltic coating on the exterior surface of the concrete masonry would form a vapor-retarding membrane on the wrong side of the insulation. This could cause the concrete masonry or the insulation to become saturated during the heating season as a result of condensation. If two vapor-retarding membranes were installed, one on the interior side of the insulation and one on the exterior side of the block, then a situation could develop where these membranes trap moisture within the concrete masonry and insulation. Any moisture that gets between these two membranes would take a very long time to dissipate. Despite all this, if you still decide to use a water-repellent coating on the exterior surface of the backup wall, I recommend using a coating with a high water vapor permeance.