Masonry Magazine October 2006 Page. 38
DRAINAGE
The major concern of an inch or less cavity wall is the drainage of moisture and the efficiency of ventilation. If we can keep the air moving and the mortar dams to a minimum, we can keep the potential for water passing back to a minimum. These once again are residential projects, and the sophistication of the system has to match the budget of the builder.
In a thin cavity, the priority is to create a space with a drainage plane throughout the wall, such as with a rain screen system or use a common technique in below-grade and create "chimney" drains matched to the weep holes. The chimney drain will provide a positive drainage path for liquid moisture to move freely to the weep, while also disconnecting the mortar dam that will occur so that air movement is assured throughout the cavity. The chimney system will not prevent the touching of mortar to the inner wall, but this is a matter of economic solution more than anything.
Filter cloths have been suggested as the drainage medium in a brick cavity, in lieu of normal weep vents and a drainage device. However, fabrics clog over time and the drying of the wall is just as, if not more important, than drainage. Fabrics have very low levels of air penetration especially when a 10-foot wall of brick sits on top of them. A minimum of 1/4 inch is necessary for convection (movement of air) to be sufficient for drying to occur, and a fabric can let liquid moisture out but is insufficient for air movement. Some parts of North America require a greater space for air movement, such as in Western Canada and the Pacific Northwest, where between 1/2 inch and 3/4 inch is required by building codes.
In order for any of these systems to work, we have to have a plane for moisture to drain. A weather barrier, building wrap or an air barrier will suffice for this. All of these systems expect that the wall will have a moisture impermeable plane on the
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