Accommodation and protection against moisture infiltration, in the form of liquid water and water vapor, is commonly considered the most important property of any new construction project. Controlling rainwater infiltration throughout the enclosure generally takes precedence, since it’s considered to be the most critical to ensuring a durable building.
There are 3 fundamental categories for controlling rainwater for above-grade walls: storage, perfect barrier, and rainscreen concepts.
As design and construction practices evolve over time, so have the methods of controlling moisture. For example, historic mass masonry walls relied on the sheer mass of multi-wythes of masonry materials to absorb, store, redistribute, and release moisture with time.
Today, moisture control in masonry cavity walls commonly relies on the rainscreen principle, which integrates multiple lines of defense, including an air space for drainage and drying. This method of moisture control has an effective performance track record.
Specifying skilled, qualified craftworkers that are trained in best practices for controlling moisture is key to the success of your project. This might include practices like proper mortar application in head joints to ensure water sheds off the veneer and mitigating obstructions within the air space like mortar fins and droppings to allow the assembly to drain and dry.
The Rainscreen Principle
The rainscreen principle’s assembly characteristics include the following criteria:
Claddings such as a masonry veneer are designed and constructed to protect against bulk liquid water, typically rainwater, from entering the wall assembly. That’s why they’re often referred to as a water shedding plane. It’s commonly accepted, however, that some moisture will penetrate the cladding. Therefore, wall assemblies require accommodations for evacuation of that moisture. One of the most effective ways to resist moisture is to ensure all mortar joints, especially head joints in the brick veneer, are properly filled. Using skilled and qualified masons who are trained in the construction of masonry veneer is critical to wall assembly performance. Specifying advanced training, like IMI’s Flashing program, is one way to ensure the level of quality on any project.
Air space behind the cladding provides a gap for moisture drainage while also allowing for air circulation to promote drying.
Weeps/vents extend through the cladding to accommodate drainage of rainwater. They’re coordinated with the flashing systems to direct the rainwater to the exterior of the assembly and are typically located at interface details like base-of-wall, window heads, windowsills, shelf angles, top-of-wall, and dissimilar cladding transitions.
Integrating weeps/vents only on top of the flashing system is referred to as a “vented” rainscreen, which provides some air movement within the air space for drying. However, to achieve enhanced drying performance within the air space, additional weeps/vents are located where the air space terminates, like at the top-of-wall and underside of shelf angles. This inlet and outlet weep/vent strategy provides increased air movement within the air space. This effective strategy is referred to as a “ventilated” rainscreen.
Water Control Layer
A water control layer, or drainage plane, is located within the assembly and provides separation between the wet zone and dry zone of the enclosure, which is commonly located on the exterior of the back-up wall. The water control layer also commonly serves as the air control layer, aiding not only in rainwater control but also vapor control, energy performance, etc.
The vapor permeance of the water and air control layer is determined by a qualified professional based on many variables, like interior/exterior environmental conditions, the properties of the various materials within the assembly, and their respective location within the assembly. Ensuring continuity of the various control layers is critical to the performance of the building enclosure.
For a rainscreen wall, including a contemporary masonry cavity wall, a fundamental building science principle for achieving effective rainwater control is the 3 D approach. In relative order of importance, the 3 D approach includes the strategies of deflection, drainage, and drying.
Deflection strategy refers to mitigating rainwater moisture load that’s deposited on the first line of defense of the enclosure surfaces, or water shedding plane. This ultimately reduces the quantity of moisture that penetrates through the assembly to the last line of defense at the water control layer.
For above grade walls, this strategy can take the form of architectural features and effective detailing based on environmental exposure conditions. Macro strategies include integration of overhangs, canopies, projections, cornices, etc. Micro strategies include coping and sill overhangs with proper slope, drip edges, drip cuts, and kerfs. Deflection strategy can also play a critical role in mitigating the risk of enclosure staining.
Drainage strategy for above grade walls relies on an air space to accommodate the free drainage of water that’s entered the assembly so that it has the capacity to egress to the exterior. It’s important that the water control layer is continuous, properly lapped, and integrated with the flashing systems. For wall conditions where masonry materials may periodically extend partially below grade, as in the case of a sloped building site, the strategy of drainage is implemented to ensure proper slope of the site to drain water away from wall. The integration of free-draining materials like drainage mats or granular fill below grade mitigates the moisture load on the masonry materials extending below grade.
Drying strategy relies on an air space to accommodate the free movement of air within the space to facilitate the rate of drying moisture within the assembly.
These fundamental building science principles and moisture control strategies are integrated into many of IMI’s conceptual details. These details are intended to be used as a guideline and modified as necessary by a qualified professional based on project specific conditions.
According to the national masonry model code, it’s the building designer’s responsibility to indicate joint type and location of masonry movement joints on the project drawings. A brief citation in the project specifications doesn’t provide enough information for masons to locate movement joints. For example, what happens at a CMU shear wall, or around outside corners with long clay brick runs on both sides of the corner?
The most common masonry movement joints are called “control joints” and “expansion joints.” Control joints accommodate movement in cement-based masonry materials like concrete block, concrete brick, and cast stone veneers. Expansion joints are typically found in masonry assemblies with characteristics of expansion, like clay brick, calcium silicate veneer units, and natural stone.
There are applications where masonry movement joints can be both vertical and horizontal. Horizontal joint are located at shelf angles, top-of-wall, between differentially moving fields of masonry, and to accommodate differential movement of masonry veneers anchored to shrinking wood structures.
It’s common for concrete block walls to have control joints spaced approximately 24’-0” on center. There’s an engineered method, however, that minimizes or even eliminates control joints. Sometimes this alternate method is used for constructability reasons or to solve tricky wall configuration scenarios. Contact IMI for more information or support using the alternate engineered method.
For guidelines on location expansion joints in clay brick veneers, refer to BIA Tech Note 18A. Note that when switching clay brick veneers to concrete brick veneers, the movement control strategies change profoundly and the BIA recommendations don’t apply. Often, concrete brick veneer assemblies have added accessories to help manage crack control and closer control joint spacing recommendations. Contact IMI for guidance.
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