Rainscreens Basics

A building’s exterior serves as a frontline defense against rain, snow, and ice. But water is tricky because it moves along surfaces and finds its way into cracks. Throughout the year, buildings can be exposed to rain and wind, which pushes water in random directions across the exterior. Snow, ice, and even dense fog can impact the exterior, and when that moisture turns to liquid, it will inevitably find a way to seep into whatever exterior material the building has as cladding.

Building exteriors are not designed to be 100 percent watertight, and even if that is a design consideration, building materials crack over time, others are porous, and many require sealants in between sections. For example, concrete and brick easily absorb water, and in environments with cold winters, the freeze-thaw cycle can mean that these materials will crack—and water will find a way to get into those cracks. Other exterior materials are at risk too. Metal panels, for example, require sealant to form a waterproof surface, but sealants can degrade when exposed to heat, sunlight, and freezing temperatures. Wood panels are also susceptible, whether through cracking, breaches in the sealant, or warping. In short, it is generally understood that no matter what the exterior design or material of a building, some water will likely make it past the first layer. The trick is making sure it does not get to the wall assembly.³

Rainscreen systems, which include exterior panels (cladding), a small drainage gap, and a WRB, prevent water from reaching the building structure. As noted above, while exterior panels provide that first line of defense, they are limited when it comes to being waterproof. Their job is to deal with water that hits the surface and direct it off of the building exterior. Some precipitation may simply bounce off the panel. Some precipitation may move sideways across the cladding surface from wind or downward from gravity. What is important is that moisture be drained off of the surface and directed away from the building.

While siding and rainscreens serve as a first line of defense against water, most builders understand that at some point in the building’s lifetime, some water will make its way past the rainscreen. Different rainscreen systems handle drainage differently, but for many, water that does make it through the cladding is then directed so that it runs along the backside of the cladding and then out through a space at the bottom of the wall. This air gap is critical in that it provides a way for the water to escape, and it also helps the wall dry.

Underneath the rainscreen and on the other side of the air gap, a WRB provides the next physical layer of protection against moisture and helps keep moisture away from the wall assembly. For buildings that have sheathing, the WRB is installed between the siding (rainscreen) and the sheathing; if there is no sheathing, it is installed between the siding and the studs.

Shown is an example of a rainscreen system, including the cladding, air gap, and water-resistant barrier (WRB).

Rainscreens as Part of a Moisture Management Strategy for Multifamily Residential Projects

When we talk about rainscreens and moisture management systems for multifamily projects, one of the more challenging issues arises in projects with multiple exterior cladding styles that need to be seamlessly integrated. While the exterior aesthetic is one side of the story, the other is that these different exterior styles often require different cladding assemblies. So, while the WRB base layer typically stays consistent across the project, cladding assemblies can change, and that means additional steps during installation and opportunities for error.

A good example of this is an exterior that includes both fiber cement lap siding and fiber cement panels. Fiber cement lap siding can attach directly to the WRB, but fiber cement panels need to have a furring system under it. Often, both materials are used on the same multifamily project, and that means that the project requires labor, time, and material costs to install the furring system for the fiber cement panels. In addition, the furring means that assembly thickness will vary, and that impacts detailing between the different materials. This, in turn, can make moisture management even more difficult to address. We will talk more about this later when we discuss the role of exterior trim in water-intrusion management systems.

WRBs come in a wide assortment of materials and designs, and each manufacturer has its own take on the product. For example, the material may be a type of building paper, plastic, asphalt felt, rigid foam insulation, or any number of other options. Most WRBs are vapor permeable, which means that they allow the wall assemblies to dry when necessary.

Each exterior rainscreen cladding material and interior WRB tends to have its own set of challenges, whether in terms of how well it repels and drains water, how the exterior panels are attached to the building, or how well the WRB works at attachment and penetration points. Whatever the case, installers need to be familiar with the product and know how to keep it watertight.

Detailing is impacted when a single project has different material thicknesses.