The Two Types of Rainscreens:

Many are not aware that rainscreen systems come in two distinct types: Drained/Back Ventilated, and Pressure Equalized. Both systems have applications in commercial and residential construction provided they are properly specified and constructed for the application in which they will be used.

Drained/Back-ventilated Rainscreens:

The current system employed by most system manufacturers is drained and back ventilated. This is a worthy system provided you have reasonable workmanship on the air/water barrier. It involves the assumption that water enters the cavity in limited amounts and is prevented from entering the building by the waterproofing. Additionally, the moisture can easily dry since the cavity is vented, causing the water to simply evaporate.

The system in force in most panel installations does not pressure equalize because of the upward or downward or sideways flow of air in a large cavity. Specifically, these large volumes of cavity space behind the panel system do not allow rapid equalization of pressures (if equalization occurs at all).

For example, if the pressure is 20 psf at the top of the wall and 15 psf at the bottom of the wall, there will probably never be 20 psf pushing back out of the cavity joints because the pressure simply migrates to a lower pressure point in the large cavity.

Therefore the planned scenario is that at least small amounts of water will enter… but then dry out due to the ventilation that is free flowing and available.

Pressure Equalized Rainscreens:

In contrast, the pressure equalized system differs in that the cavity will be broken up horizontally as well as vertically so that individual (smaller) panel areas can pressure equalize. The amount of venting is then carefully calculated based on the cavity volume.

The benefit is that pressure can build up rapidly in the cavity and push back at the same pressure that it is pushing in the vents/seams and gaps, resulting in less water (or no water) entering the system.

Another advantage is that the pressure presses equally on both sides of the panel resulting in a theoretical Net 0 psf on the panel system itself. Consequently, the building design pressure can be applied to the wall behind the panel system. Although this is potentially true, building codes and safety factors require that you design fastening systems able to withstand the design pressures.

Metal Construction Association (2006). "Understanding 'The Rainscreen Principle"