The rainscreen principle is a design concept that works to prevent the penetration of running or wind driven rain water through that part of a window or wall system exposed to the exterior. Pressure equalization is a method of making the pressure on both sides of the rain screen, the same so that rain water will not be sucked or drawn through the screen by pressure differences. A rain screen must be used to achieve pressure equalization and conversely pressure equalization will depend upon an effective rain screen.
In 1953, the Alcoa Building (now the Regional Enterprise Tower), become the first commercial building using the rainscreen principle. This is a 30 story tower in Pittsburgh using 1/8” aluminum panels with open, labyrinth joints.
In the early 1960’s, research was conducted in Norway on rain penetration of windows and walls. Mr. O. Birkeland published the first article referring to a “rain barrier”.
In Birkeland’s article he “suggested that venting the cavity behind the screen would equalize the pressure on either side of the screen and essentially eliminate air pressure differences as a rainwater penetration force.”
In 1971, the Architectural Aluminum Manufacturers Association (AAMA) published the first guide for pressure equalizing design.
The objective of a rain screen wall is to prevent rain penetration.
For wind driven rain to penetrate a wall assembly, three conditions must exist:
If any one of these three conditions is eliminated, rain penetration will be significantly reduced or eliminated.
Rain water can penetrate the wall of a building in the following ways:
Gravity is the force that pulls water down the face of the wall and into openings.
Kinetic energy is generally referred to as wind-driven rain. Wind loads on buildings are subject to many variables including wind direction, gusts, building geometry, and surrounding conditions. This force is also the most associated with air pressure differences.
Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.
Capillary is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to external forces like gravity. The forces involved are a combination of the surface tension of the water and the adhesive forces between the water and the adjacent materials.
Air currents are very much like the kinetic energy however without the wind-driven rain.
When one side of the cladding system has a positive pressure and the other side a negative, water will be pulled to the negative side.
While all six are important factors, the first five have been dealt with by manufacturers in the design process. The last item, pressure difference, is why glazing professional like Ventana have gone to pressure equalized systems as illustrated in the following detail.