A number of buildings being designed these days call for dampproofing on the cavity side of concrete masonry backup walls. In addition to the dampproofing, vents often are added to the top and bottom of the walls. The dampproofing and vents can add quite a bit of cost to the wall system. We have designed many buildings with a 14-inch cavity wall system without dampproofing or vents. To date, we are not aware of any problems with this type of construction. What has prompted this change in design? What are the pros and cons of dampproofing and venting?
Designers often specify dampproofing on the cavity face of the concrete masonry backup wall to prevent water from being absorbed by the concrete masonry. Water can reach the face of the backup at mortar bridges or other obstructions of the cavity. Such cavity obstructions allow water to bridge across the cavity and run down the face of the backup wall. In a well-built wall with an open cavity, dampproofing on the face of the block is not needed. The dampproofing, however, can provide additional protection. Vents are sometimes installed at the top and bottom of these wall systems in an attempt to create a rain screen. In the rain screen concept, the vents allow air to flow into the cavity and equalize the pressure on both sides of the veneer wythe. This eliminates the pressure drive that can cause water to be drawn into the masonry wall. For a rain screen wall to function properly, however, the area of intentional openings in the veneer should be approximately 10 times the area of unintentional openings in the backup. It is also essential that the cavity be divided into compartments by providing vertical and horizontal barriers within the cavity. Vertical barriers are especially important at corners; however, intermediate barriers are typically provided. The relieving angles typically serve as the horizontal barriers if adequately detailed. Unless these additional steps are taken to create a rain screen, vents generally are not needed at the top of the wall. Vents also are installed in cavity walls to help dry the walls quickly after rains. Ventilation of cavities is often a consideration in glazed brick masonry where dissipation of moisture is important in preventing spalling problems. Depending on the type of vent used, however, vents along the top of the cavity may increase the quantity of water entering the wall. Water running down the wall can flow into these openings with or without a pressure drive. I usually do not recommend using rain-screen walls in areas where masons are unaccustomed to building in this manner. A properly designed and constructed drainage wall system will work effectively to resist water penetration.