There is an expression, 'Those who don't know history are doomed to repeat it.' This quote applies very well to building enclosure wall assembly failures. The majority of wall assembly failures can be attributed to deficient design for rainwater management.
The concept of a rainscreen for rainwater management is nothing new. References to the pressure equalized rainscreen concept can be found as far back as 1963 in Canadian Building Digest #40 (CBD #40). In 1963, G.K. Garden authored CBD #40 which discussed the rainscreen principle and the importance of air barriers as a component of the rainscreen principle. The digest also included the prophetic words, 'It is not conceivable that a building designer can prevent the exterior surface of a wall from getting wet nor that he can guarantee that no openings will develop to permit the passage of water.' Fifty-two years later, designers, developers, and contractors continue to ignore this fact, leaving their projects vulnerable to water damage. A properly designed and constructed rainscreen system can avoid the headaches of wall assembly failures due to water damage.
The rainscreen concept follows a 3D approach to rainwater management:
A drained/back vented rainscreen is comprised of exterior cladding, with a vented and drained space behind the cladding, and an air/water barrier. The exterior cladding deflects water, and the vented drainage space allows incidental moisture to drain or dry from the wall system. A pressure equalized rainscreen includes all of the components of a drained/vented rainscreen and also adds compartmentalization of the drainage space, which enhances the deflection function of the rainscreen.
The vented compartments in a pressure equalized rainscreen use the concept of pressure equalization to reduce the driving force for water to enter behind the cladding. When wind blows on the building, the vents in the rainscreen cladding allow air to enter and pressurize the compartment, which equalizes the pressure in the compartment with the exterior wind pressure. With equal pressure on both sides of the cladding there is no air pressure differential to draw water into the wall system. The air barrier is a key component to the pressure equalization function since it is impossible to equalize a compartment if it is not air tight.
The primary difference between rainscreen and non rainscreen exterior cladding is the provision for a defined drainage space and vented drying. With non rainscreen systems, deflection is relied upon as the primary method to control water intrusion. Although many non-rainscreen systems such as stucco cladding include a water resistive barrier, flashing, and weeps, there is little if any ability for water to actually drain to the weeps or dry from the wall. A common misunderstanding is the perception that the water resistive barrier will prevent moisture damage to the wall. Water resistive barriers are water resistive, not waterproof and prolonged exposure to moisture can result in water intrusion beyond the water resistive barrier. Moisture intrusion beyond the water resistive barrier can result in wall assembly failure, particularly in wet climates.
Both vented/drained rainscreens and pressure equalized rainscreens can offer excellent protection against water intrusion and related damage to moisture sensitive wall components. Vented / drained rainscreen assemblies increase the drainage and drying potential to remove water from the wall assembly. Pressure equalization of the rainscreen goes one step further by also reducing the driving forces for water to penetrate beyond the exterior cladding. The rainscreen design approach offers a time-tested, cost-effective solution to rainwater management.