A Clean Air Space

The purpose of a clean air space is to be sure that water can't get across to the backup wall. The Brick Industry Association (BIA) recommends, and the Council for American Building Officials (CABO) One and Two-Family Dwelling Code requires, a minimum 1-inch air space between the brick veneer and the backup wall system. Often, however, this 1-inch air space, which is little more than finger-room for the mason, is reduced because of simple variation in the materials - sheathing that is not properly attached, framing that is out of plumb, or even variation in brick size. With a larger air space, mortar is less likely to bridge the air space, and masons can remove mortar more easily from the back face of the brickwork. Because a larger air space is less likely to become clogged, more and more designers and masons are including a larger 2-inch air space whenever possible. It's also important to keep mortar droppings to a minimum. Dropped mortar can block weepholes, and can also cause bridging that allows water to travel to the backup wall. Because some bridging across mortar droppings is inevitable (particularly with a small air space), it's also a good idea to install housewrap or #15 asphalt felt over the sheathing on wood-framed houses to help prevent moisture migration. If housewrap or felt is installed, the BIA recommends that it be lapped at least 8 inches over the through-wall flashing (Figure 3).