by Steven Fechino
Rainscreen systems, like most of today’s wall assemblies, have become more complex as designers strive for greater energy efficiency and building sustainability. This article will provide an overview of the two types of rainscreen wall systems and typical design characteristics, and offers some direction on how to design highly effective and sustainable rainscreens.
However, it is important to understand rainscreen walls are design intensive, so specifics for creating an individual system are beyond the scope of this article. While all rainscreen wall systems share the same general characteristics, there is no one-size-fits-all design.
The rainscreen principle
The basic definition of a wall designed using the rainscreen principle is that it has two distinct barriers to the outside elements separated by an airgap or cavity. The outer barrier provides the aesthetics of the building envelope and sheds or controls most, but not all, of the moisture hitting the building veneer, while the inner component serves as the final moisture barrier. The cavity provides a capillary break to prevent the flow of water from the outer to the inner barrier. It is also vented to allow pressure equalization between the cavity and the outside air. According to building science consultants, the capillary break should be 4.7 to 9.5 mm (3/16 to 3/8 in.) wide to provide proper drainage and ventilation.
The two types of wall systems utilizing the rainscreen principle are:
- drained/back-ventilated (DB/V); and
- pressure-equalized (PE).
Both use the basic design principles described earlier, but DB/V rainscreens are built with the expectation the cladding will leak, and no deliberate attempt is made to minimize the effects of wind on pressure equalization. These rainscreens rely on unrestricted air movement within the air cavity to drain and dry moisture and vapor, and the cladding fastening system must be designed to accept 100 percent of the wind load.
In the author’s experience, PE systems typically do not perform as well as DB/V rainscreen walls. However, they offer the architect an alternative option when design challenges arise. In the PE system, every effort is made to minimize or eliminate leakage through the joints in the cladding, and the wall assembly includes air dams to separate the cavity into multiple drainable compartments between the inner and outer barriers. These compartments are designed to limit water penetration during periods of extreme pressure differentials and to provide rapid pressure equalization, so water is not forced into the substrate. With these types of walls, the cladding fastening system may be designed to accept less than 100 percent of the wind load.
A masonry cavity wall is not considered a rainscreen even though it has inner and outer barriers separated by an airgap. The masonry veneer is designed to be far more water- and air-tight than a rainscreen (but not 100 percent water- and air-tight) and is not designed with the extensive ventilation or pressure-equalization capabilities of a true rainscreen wall system.
Unfortunately, the definition of rainscreen has become a bit blurry during the past few years. Rainscreen used to refer only to the façade material of a wall built using the principle described earlier. The façade functions both as a design element and as the outer barrier intended to protect the building from weather or manmade elements. Weather, of course, is wind, rain, humidity, temperature, and sunlight, while manmade elements may include water from sprinklers, snow pushed against the structure, flying debris, and noise.