Controlling Moisture in Masonry

Resolving the number one cause of structural deterioration in buildings
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Proper Masonry Design and Installation for Moisture Management

Moisture management ranks as a top priority in a wall, and appropriate design and installation techniques will go a long way to eliminating the potential for health, legal, and financial devastation caused by excessive moisture in the building envelope. While it is best to keep moisture from entering a wall in the first place, that is probably an unlikely occurrence. Modern designs are geared to thin wall construction and, to some extent, this allows buildings to leak. Besides precipitation, moisture can enter masonry walls from several different sources, including capillary action, water vapor, and groundwater. The objective is to control the water that enters a wall and provide a way for it to drain before it has a chance to do any damage or penetrate further. The National Concrete Masonry Association maintains that successful moisture mitigation in concrete masonry walls involves several techniques, including flashing and counter flashing, weeps, vents, sealants, water repellents, post-applied surface treatments, vapor retarders, and crack control measures. All components should have redundant use, with the preferred approach to controlling moisture being a four-level line of defense, including surface protection, internal protection, and drainage and drying. The strategy here is that a wall’s water tightness will still be preserved if one of these systems fails.

Detailing techniques vary by type of masonry unit. The following are three of the most common masonry scenarios:

  1. Full Bed Depth Veneers:In a cavity wall, moisture control relies on gravity and an unobstructed 2-inch airspace to get water down to the flashing and the weeps. This is the basic rainscreen principle wall.
  2. Single Wythe Walls: A single wythe wall is a stone, brick, or concrete wall that is one masonry unit thick. Single wythe walls offer the economic advantage of serving as the structural system with multiple finish options on the exterior and the interior. Single wythe walls do not require the backup of a traditional cavity wall construction, but in order to provide full protection from the elements they must be carefully detailed and constructed
  3. Manufactured Stone Thin Veneer: An increasingly popular masonry solution is manufactured thin veneer, which refers to a lightweight, flat-backed, thin surface product that is applied directly to a solid facing. Thin veneers average in thickness from 1-inch to 2-inches and, per International Code Council regulations, must weigh 15 pounds or less per square foot. Thin veneers can either be directly adhered or attached to a mounting system. According to the Masonry Advisory Council, a proper model for an adhered exterior veneer that provides adequate moisture control uses Oriented Strand Board (OSB) as sheathing on the outer face of the studs, followed by two layers of building paper. A drainage mat is then installed to drain any water that has penetrated to the flashing, which in turn sends it to the building exterior via the weepholes. Atop the drainage mat, a galvanized expanded metal lath, or diamond mesh, should be attached as it will hold the veneer to the studs. A parging backing, which is a thin coat of mortar to provide a smooth surface for masonry and seal it against moisture, should be applied onto the mesh. Mortar is applied to the back of the thin veneer and pressed on to the parged surface.

Isometric view of thin veneer in a grouted installation over wood framing.

Photo courtesy of Oldcastle® Architectural

Isometric view of thin veneer in a grouted installation over wood framing.

 

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Originally published in Architectural Record.

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