Cavity wall designs continue to change and evolve with the use of air/vapor barriers, improved brick/wall ties, various types of rigid and spray insulation and control/expansion joints. One thing that has remained constant is the importance of effective drainage. From the early days of cavity wall construction, architects and masons saw the need for effective drainage.
For many years, pea gravel was the product of choice for drainage and keeping weep holes open and free flowing. In time, it was recognized that pea gravel only served to dam the mortar droppings up higher in the cavity and did not maintain an open path for water passage. Eventually, synthetic mesh materials with a dovetail shape and 90% open weave were introduced to catch and break up mortar droppings at two distinct heights, ensuring a clear path for water to migrate out of the wall.
Another major component of the "drainage system" is the weeps, which are located at the bottom of each cavity section and spaced either 16 or 24 inches on center. Once again, the evolution of weep materials has gone from cotton sash cords and plastic tubes that are very easily clogged by insects and debris, to full head joint products made of plastic, aluminum or synthetic mesh. These larger weep products provide greater protection from clogging and also allow more drainage area. Completely open head joints are also used on occasion.
Recently, the idea of adding vents at the top of each cavity compartment has come into play. Many times these vents can be the same material as the weep vents. Working in conjunction with the drainage system, these vents create an airflow avenue to further ventilate the cavity area. This additional airflow provides many benefits including drying out the components within the cavity much more quickly, which can dramatically reduce the opportunity for mold formation. Independent testing has shown that these vents can increase airflow through the cavity by up to 46%.
A "drainage system," including an effective dovetail shaped mortar collection device, weeps at the bottom, and vents at the top of the wall, should be a major part of any cavity wall design, especially when used in conjunction with an air/vapor barrier system.
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