Preventing moisture penetration

Masonry Construction, Sept, 2003 by Norbert V. Krogstad

Q I have seen many warehouses built with single wythe concrete masonry walls. How does this wall prevent water from penetrating into the interior?

A Water penetrates the untreated exterior or face of concrete masonry walls just as it does the exterior face of clay masonry walls. This fact is the reason why cavity walls are popular in masonry installations.

In a cavity wall, water that penetrates the exterior wythe is prevented from reaching the interior wythe because of the open air space between the two. When single wythe masonry walls are used, a coating must be applied on the face to stop water from penetrating in the first place, or the walls must be designed so that the cells of the concrete masonry units function as a drainage cavity.

Probably the most common approach with single wythe concrete masonry walls is to use a sealer or elastomeric coating on the exterior face to prevent water penetration. This coating generally needs to be capable of bridging small cracks that may form in the exterior face of the wall system.

In some cases, water repellent may be effective, but there must not be any significant voids, separations, or cracks in the concrete masonry units must have a relatively fine texture.

The wall system must contain control joints that are spaced closely enough to prevent shrinkage cracks from occurring. Water repellents do not prevent moisture penetration through most shrinkage cracks during a wind-driven rain.

For a single wythe masonry to function as a drainage wall with the water draining in the cells, it must not be reinforced and must be constructed using face shell bedding. An integral water repellent admixture must be applied to greatly reduce the absorption of the units.

If an integral water repellent is not used, moisture is absorbed by the units and prevents the cell from functioning as a drainage cavity. With an effective water repellent admixture, moisture that penetrates the exterior face of the wall flows through the exterior face and travels directly into the cell or onto the web of the block. In most cases, water does not flow across these webs. The small amount of water that may bridge the webs is generally stopped by the interior face shell mortar joint and flows down into the cell. Water that penetrates the exterior travels within the core and down the flashing.

The cells must not be blocked so that they can function as a free draining cavity. Also, the flashing must be carefully designed and constructed to direct and divert the water.

Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.