Aren't all cavity walls rain screen walls?
No. Most cavity walls are not rain screen walls. A rain screen wall is a cavity wall designed to quickly equalize pressure between the exterior face and the interior face of the exterior wythe of masonry. With equalized pressure, water entering the cavity is not forced through the wall. Any water penetration of the interior wythe will occur only as a result of gravity flow and capillary action. However, to create an effective rain screen wall, it's necessary to partition the cavity and provide vents in the exterior masonry wythe. The cavity must not continue around corners and should be partitioned at every floor line. The cavity partitions and the backup wall should be as airtight as possible. The exterior wythe's face, on the other hand, should contain several vents to allow the air to flow quickly into the partitioned cavity space. The vent area required for pressure equalization is highly dependent on the air leakage area through the backup wall. If, due to poor construction, poor detailing or damage, the air leakage area of the backup wall is more than anticipated, wall-pressure equalization could be reduced significantly or eliminated. I would consider using rain screen walls only if installed by a contractor experienced with this type of construction and if ongoing observation occurs during construction. When built incorrectly, cavity partitions may actually act as bridges for water to flow between the veneer wythe and the backup. Also, the vents along the top of the wall sections can let water into the wall system, which may create additional problems. However, when built properly, rain screen walls are very effective in resisting water penetration.