There are two types of rainscreen systems: drainable assemblies and pressure-equalized rainscreen systems.
Drainable assemblies, often referred to as a “drained cavity wall” or “drained/backventilated” (D/B-V) system, take into account that some moisture will be driven past the outer skin and allow this moisture to drain harmlessly away through the use of a drainage plane.
Drainage assemblies consist of two layers separated by an airspace. The outer layer is the cladding that sheds that majority of rainwater, while the inner layer is usually the exterior surface of the structure, which must be protected by a weather barrier to shed any water that does get past the cladding. The air space serves as a place for water to drain down to the appropriate base flashings, while also allowing air to circulate behind the cladding. Additionally, this back ventilation helps dry the wall by promoting evaporation.
Meanwhile, pressure-equalized rainscreens (PERS) rely on a series of pressurized compartments that usually consist of a drainage plane vented at the top and bottom and closed at the sides. These systems are designed to eliminate or significantly reduce the water that is allowed past the outermost layer of the envelope, and also manage any moisture that does get past the outer layer.
When wind blows against a wall with a pressure-equalized rainscreen, the compartments fill with air. Because the sides are closed, and the airflow through the vents is limited, the compartments are only able to fill up until the air pressure matches the pressure of the incoming wind. Essentially, the air inside the compartment pushes back, and when the wind subsides, the pressurized air in the compartments will flow out the vent until the pressure inside again matches the outside pressure. As long as the pressure on each side of the cladding is the same, there is no driving force for air and water to penetrate the cladding.