Q I have heard people recommend using weep vents at the bottom and top of walls to help dry them out following a rain.

Does this product have any value? Will the weep vents commonly used in masonry walls allow sufficient airflow to significantly change the rate of drying following rains?

A In many parts of the country, considerable amounts of water can be absorbed by brick masonry over the rainy portion of the year. Some of this moisture evaporates into the air within the cavity, making the air there very humid.

Moisture from hot and humid cavity air can migrate through the wall system and condense on cool interior surfaces resulting in deterioration, especially on wood or paper materials, that can support decay and mold growth. This problem is acute where vapor retarders contain many voids and discontinuities.

Efflorescence also may appear on the surface if the walls remain wet for extended time periods. The faster the walls dry out following rains, the less time available for salts and soluble compounds within the mortar to be carried to the surface.

Drying out masonry through the venting of cavities can significantly reduce these problems. A study published from the 8th Canadian Masonry Symposium in 1998, Vent, Ventilation, and Masonry Veneer Wall Systems by J.F. Straube and E.F.P. Burnett, found a significant increase in the rate of drying when vents were used in masonry walls.

The authors stated that open head joints worked fairly effectively in venting the wall if they were placed at 24-in. o.c. They discovered, however, that commercially available veneer vents restricted airflow to 5% to 15% of an open head joint. They concluded that although vents can be effective in drying walls, more vent area is needed for them to work effectively.

To provide measurable benefit to masonry veneer walls, the authors recommended using open head joints at 24-in. o.c. at the top and bottom of wall sections. They also favor using clear air spaces in the cavity over 1.5 in.

The use of open head joints at the base of the walls work reasonably well since water entry into weeps are handled by properly installed flashing. However, if the flashing is not watertight, weeps can let more water enter the walls, resulting in leakage problems. Therefore, as with any flashing system, careful attention should be given to forming watertight splices and corners.

To facilitate air movement by natural convection, vents are needed at the top of the wall as well. With vents at the top and bottom of walls, warm air rises within the cavity during mild weather or when the wall is exposed to direct sunlight. This rising current draws air into the vents at the base of the wall and out of the vents along the top of the wall, as shown in Fig. A. In high-rise construction, vents are needed above and below each level of shelf angles to take advantage of convection to help dry these walls.