Differences Between Rainscreen Walls and Cavity Walls

Words: Dan Kamys

Moisture Management

By Steven Fechino

[caption id="attachment_13432" align="alignnone" width="600"]Rainscreen veneer terra cotta during installation. Rainscreen veneer terra cotta during installation.[/caption] Rainscreen walls have had a few different definitions over the years. Rainscreens can be designed with products and configurations that assist in creating both pressure-equalized and non-equalized or drained/back ventilated systems — all still in line with the correct definition, but resulting in some industry confusion.

What Makes a Rainscreen?

The term rainscreen originally referred to the outer shell of the façade, usually an aesthetically based material that functioned as a barrier to protect the building from weather or man-made elements. Weather, of course, is wind, rain, humidity, temperature and sunlight, whereas man-made elements can be anything that impacts the structure, such as sprinklers, surplus snow pushed against the structure (common in northern states), flying debris and noise. There are many other elements, but the point is that the rainscreen is a protective barrier. Its components are engineered to function as a complete system: the rainscreen material, the type of channel and clip arrangement used to fasten the rainscreen veneer, the dimension of the air gap, the three-dimensional drainage mesh, and the air barrier and flashings. Rainscreen, when defined as the façade material, is typically a terra cotta, precast, composite or metal panel, or even a very thin stone with lightweight structural backing. The lightweight material is a composite fastened independently to a framework that supports the façade to the substrate and creates the mounting attachment points for the façade material. Rainscreen façade material can be installed with a gasket-sealed, material-to-material joint (horizontal and vertical joints) or with open material-to-material joints (horizontal and vertical joints). The term pressure-equalized system has led some to believe that sealed walls will be drier than those that have open joints. In comparison to cavity wall construction, sealed walls are definitely the preferred choice to stand the test of time and performance, but a rainscreen wall is slightly different.

Cavity Wall vs. Rainscreen Construction

Masons are most familiar with cavity walls. While we have seen mass and composite walls, 99 percent of our veneer work has been in cavity wall construction. Cavity wall construction is simply a veneer material, separated by an airspace, sometimes insulated, and usually with an air and/or vapor barrier. Flashings usually round out the base of the walls and at the masonry opening locations, extending from the substrate beyond the face of the veneer. In cavity walls, moisture that enters the cavity has two preferred paths to keep the substrate dry: either it is moved by gravity to the flashing level before exiting the cavity, or it can become a vapor and evaporate through the face of the veneer by means of weeps, soffit vents or (in the case of terra cotta veneers) the mortar joints. [caption id="attachment_13431" align="alignright" width="300"]Rainsceen veneer terra cotta used alternatively to clad a column and ceiling of structure. Rainsceen veneer terra cotta used alternatively to clad a column and ceiling of structure.[/caption] Cavity walls connect the exterior veneer to the substrate with wall ties that span across the cavity. The individual ties have many shapes and can be installed between courses of concrete masonry units (CMU) or attached directly to the substrate, which could create moisture contact directly with the substrate wall. Using hot-dipped galvanized or stainless steel connectors will help prevent corrosion because they will be a likely path for water to follow; however, the principles of a well-functioning cavity airspace will control a majority of the surplus moisture. In comparison, the same basic principles exist for cavity walls and pressure-equalized walls: we attempt to manage the water that enters the air gap between the substrate’s protective weather barrier and the veneer or fastened veneer material, and keep the inner and outer wythe drier, thereby preventing such problems. So, what is the difference between most cavity walls and a rainscreen wall? In many cases, the cavity of a cavity wall, which is typically much larger than a rainscreen air gap, will deflect due to external atmospheric forces and material dimensional changes. Thermal exposure will form negative pressures within the cavity, possibly leading to vapor and moisture penetration beyond the air barrier into the wall interior. In short, a cavity wall is not usually a rainscreen wall, and a rainscreen wall that has a small cavity (a.k.a. an air gap) is not typically referred to as a cavity wall. Pressure-equalized walls are the basis for one type of rainscreen wall. The open material-to-material joints found in many rainscreen systems will experience pressure equalization when the external wall pressures are applied to the face of the wall and the open joints allow the air gap pressure to nearly equalize with the external pressure. It is not logical to believe that the pressures will ever be exactly the same. Equalization principles, or normal air movement, prevent pressures found in the cavity from becoming negative. These negative pressures, if present, would force moisture into the substrate walls, gaps in electrical and plumbing intersections, and areas that have exhibited displacement (either large or very small) during or after construction. In the past, rainscreen was a general term, usually used to refer to a façade material. However, the term now typically defines the entire system of either hat channel, girt or rails, air barrier or structurally fastened insulation systems where the façade material is secured to a framework independently from the attachment to the substrate wall. The joints between the individual units that comprise the façade are either sealed using gaskets or left open. Open joints can often have a semi-sealed, three-dimensional mesh with a backer material placed against the back of the façade if insulation is not present; this allows air and vapor to access the air gap.

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When discussing open joints, we are not talking about a head or bed joint that is 3/8 inch wide, left completely open. We are talking about a joint that can be configured several ways. One of the most common is to use a slightly open ship lap joint. The open joint is designed to allow only air into the air gap, and this happens when the material-to-material joint, whether horizontal or vertical, fits over the lower unit or adjacent unit, thus preventing the passage of wind-driven rain. This will also work with the rainwater that runs down the building because of gravitational forces. Due to the joint in the façade material, water typically does not find its way back up and into the wall through the ship lap joint. Another feature of rainscreens in pressure-equalized walls is the placement of limiters or dividers to create channels within the air gap to compartmentalize air movement. This involves simply installing stainless steel, composite, rubber or foam strips that fit snugly in the air gap and are taller than they are wide to limit air movement to a localized area within the wall. Pressures in the air gap (wall cavity) will require less change in the center of the walls and can be dimensioned much larger than sections surrounding the perimeter of the wall, where the compartments would require smaller sections to achieve similar results. Here is a rule of thumb: moisture, air and water always move from high to low. For instance, wet material (high moisture content) moves to dry (low moisture content); high vapor moves to low vapor (from outside to inside during the warmer months); and high temperatures move to low temperatures (outside to inside during the warmer months). With this in mind, it becomes easier to visualize rainscreens and the principles that make them function well in so many different climate zones. Air barriers and flashings are critical in rainscreen design because they are placed against the substrate and are the final defense for keeping things dry. Air barriers, when used in rainscreen systems, will be chosen with different criteria than a cavity wall, though they serve a similar function. The difference in performance is based on the pressure differentials between the air gap and the outside of the building. Air barriers must be durable, so that deflection or “pumping “does not occur and create negative pressures within the air gap. Flashing of the rainscreen wall, when designs require, would follow standard flashing procedure. The flashing should include a small drip edge — in many cases, smaller than the drip found in most cavity wall construction; flashing membrane, usually dark in color; and termination bars, sealed to the air barrier to mechanically support the flashing for the life of the building. Many rainscreen systems do not require a mortar collection device or individual weep, as the veneer is typically placed away from the substrate without mortar, allowing full drainage of the water that enters the air gap. One of the major differences of a rainscreen system versus a cavity wall system is that the layout of the veneer must be exact to keep the building lines horizontal. Shimming of the Z-girts or hat channel is sometimes required to keep a square, level and plumb façade. Having an exact layout from the beginning at all corners of the building will provide a big return for the installer. In cavity wall construction, the installation of brick and mortar allows for slight corrections to be made in order to make the proper bond. On the other hand, rainscreens do not typically offer the same compensation levels. Therefore, it is recommended to spend the extra time up front, before installation, using quality instruments to set your points. Once installation begins, repairs or unit replacement can usually be performed in the specific location by sliding and resetting the units. This is vastly different from repairing a wet-mortared brick veneer project, where stocking, sawing, cleaning and debris disposal become part of the repair. Rainscreen wall systems have been in our industry for a long time. They have stood the test of time, function as intended, offer particular aesthetics to a project and can be managed by many masons with little or no training.
Steven Fechino is engineering and construction manager with Mortar Net Solutions, makers of TotalFlash and BlockFlash. For more information, call (800) 664-6638, or visit www.mortarnet.com.
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