Technology | Moisture Control
By Art Fox
All images courtesy of Mortar Net.
As wall systems evolve, masonry contractors are increasingly being asked to install rainscreen wall systems. Masonry cavity walls and rainscreen walls share many of the same time-proven design principals that control water and prevent water damage, including a weather-resistant façade and a cavity between the façade and substrate, but there are a few key differences between them.
A masonry cavity wall is a wall type that has been around for more than a century and one masons are very familiar with. It is defined as a wall with a masonry veneer material separated from the substrate by a one-inch or greater air space, or “cavity.” The substrate can be wood or steel studs covered with panels such as plywood, OSB or gypsum, or concrete masonry units (CMUs). Flashing systems and mortar dropping collectors are part of the design, and because of increasingly stringent energy requirements, most modern masonry cavity wall designs also include continuous rigid insulation (RI) in the cavity and an air and/or vapor barrier over the substrate.
The definition of a rainscreen wall is a little harder to pin down. While the term “rainscreen” originally referred only to the outer shell of the façade, common usage has expanded the definition to mean the façade only, the complete rainscreen system consisting of all the components in a rainscreen wall, or some of the individual components. For the purposes of this article, “façade” will refer to the exterior veneer, “rainscreen” will refer to the entire system, and individual components will be called by their more common names, such as drainage mesh or air barrier.
The outermost and most visible rainscreen component is the façade material, which is usually chosen for aesthetic purposes, and which functions as the primary barrier against the elements. Façade material is typically panels made of terra cotta, precast concrete, composite, metal, thin stone, wood, or glass. Panels can vary dramatically by color, texture and size, so rainscreen systems provide a wide variety of design choices.
The façade material is fastened to a framework attached to the substrate, not directly to the substrate itself. Mounting systems can be as simple as wood furring strips, which are typically used behind shiplap-style siding on residential, multi-family or light commercial buildings, or complex metal rail systems, which are typically used on commercial and industrial buildings to support large steel, masonry or glass panels. Rainscreen façades are not designed to be waterproof. They are designed to deflect the majority of moisture and wind, with the water and vapor barrier on the substrate preventing water that gets past the veneer from penetrating the substrate.
Moisture in both liquid and vapor forms is by far the most common cause of damage to walls of all types. Masonry cavity walls are designed with the expectation that water in both forms will get into the cavity, either through leaks or humidity in the air, so the wall is designed to control and manage both forms of moisture. To remove liquid water, flashings extending from the substrate beyond the face of the veneer are installed at the base of the walls and over masonry wall openings to channel water in the cavity to the outside of the wall through weep holes. Air barriers are used on the substrate.
In pressure-equalized rainscreens, the open material-to-material joints found in many rainscreen facades allow air to move much more freely into and out of the cavity than the weep vents in a masonry cavity wall allow. Pressure-equalized rainscreens also use compartmentalization within the cavity to allow rapid pressure equalization in small areas of the cavity, rather than throughout the entire cavity as a masonry cavity wall requires. Drained/back-ventilated rainscreens do not use compartmentalization and do not have the same rapid pressure equalization characteristics as pressure-equalized rainscreens.
Rapid pressure equalization is valuable because it prevents negative pressure in the cavity—that is, pressure inside the cavity that’s lower than the outside pressure. Since water will always move from higher pressure to lower pressure areas, negative pressure will, in effect, suck moisture into the cavity. The greater the negative pressure and the longer the pressure differential stays negative, the more water will be sucked in. The more often negative pressure can be prevented or reduced, the drier the cavity and substrate will be and the less likely it will be that moisture damage will occur.
Pressure-equalized rainscreens are compartmentalized because differences in air movement at wall corners, edges, and higher up create pressure differentials that can be significantly greater than in the center of the walls. By creating smaller chambers at the areas where pressure differentials are greatest, the pressure inside the cavity within small chambers can adjust much more rapidly than it can throughout an entire, undivided cavity. It’s like the difference between deflating a child’s balloon versus a truck tire—the balloon will deflate, or pressure equalize, much faster than the tire because there’s far less air to move. Compartments are created using stainless steel, composite, rubber or foam strips placed both vertically and horizontally to fit snugly in the air gap.
In addition, rapid pressure equalization can help prevent water damage caused by wall “pumping,” a condition in which the wall is deflected by air pressure, which creates pressure inside the cavity that forces water through openings in the substrate. Pumping can occur across large wall faces and can force water through openings such as those caused by electrical or plumbing penetrations, cracks that occur over time, and even through some fastener holes.
Like modern masonry cavity walls, rainscreens frequently inc350lude insulation installed outside of the substrate, and should include flashing systems and air and/or water barriers. Drainage mats installed continuously between the façade and substrate are becoming more common and provide both added protection and cosmetic value. They include a water-resistant fabric on the outward-facing side that helps protect the substrate from moisture that gets past the façade, and they guarantee a continuous channel for water to flow to the bottom of the wall where it’s collected by the flashing and directed out of the wall. They are frequently dark colored so they hide the substrate or insulation that might otherwise be visible through gaps in the façade.
Masonry cavity walls and rainscreens both offer a wide variety of design choices, and when they’re properly detailed and installed to manage moisture effectively, they provide reliable, sustainable buildings.
Art Fox is in charge of marketing and communications for Mortar Net Solutions. He has been involved with the company and the masonry industry in various roles since 1992. During the 1980s, he ran his own construction company in Albuquerque, N.M.