RainScreens
This is a quick overview of the RainScreen concept.
Concept
In a rainscreen design the exterior surface of the wall is not considered a weather barrier as in most traditional wall systems. Think of wood siding; nothing is intended to get past the siding. In a rainscreen wall, most water is kept out but not necessarily all water. Any water that makes it through the screen is conducted down through the air space and back to the exterior. The archetypal example is a masonry cavity wall. Rather than just one line of defense, the rainscreen provides two lines of defense and the critical second line of defense is subject to only a fraction of the rain water, and it is not wind-driven by the time it reaches the inner barrier.
The diagrams above were taken from Wikipedia - http://en.wikipedia.org/wiki/Rainscreen_cladding
Components
Structural backup. Studs or CMU. Metal studs are most popular but are less forgiving than concrete mansonry. In a metal stud the gage of the steel, the type of and material of the fasteners is crucial. Structural integrity comes down to the threads of the fastener not losing their bite in the flange of the stud (corrosion and electrolysis undermine everything - can you be sure the correct fasteners are used properly?)
Air and water barrier. This needs to be monolithic. Pay special attention to joints and penetrations.
Insulation. This also needs to be monolithic with sealed joints and penetrations, and a minimum of 'thermal shorts'.
Air space. In masonry, code requires 1" min and 5.5" maximum without specially engineered tie-backs. Equalizing the pressure through weather-resistant vents is desireable. This prevents a negative pressure developing in the air space that sucks moisture in. Encourage vertical ventilation of air space by providing barriers to horizontal movement.
Rainscreen. There are many types of rainscreen: brick veneer, 3-5/8" calcium silicate masonry units, 1-3/8" calcium silicate panels on a clip system, terra cotta, stone veneer, fabricated panels of many types.
PROs
Generally more forgiving weather barrier for the building
Better energy performance
Can be a 100 year solution
Can be lighter weight
CONs
Penetrations, doors and windows, are critical. Repairs much harder.
Concept doesn't have decades of history to show up any long-term weaknesses (during the past half century there have been plenty of new ideas that didn't pan out)
More expensive
CONCLUSION
A rainscreen wall is becoming the expected, modern solution to exterior walls. The benefits outweigh the disadvantages. Where budgets allow and capable workmen can be counted on to implement the design, the rainscreen is the best solution. Bear in mind that it will take more design effort to create a good rainscreen, and nothing can be readily delegated to the contractor, unless he is a specialty rainscreen contractor with proven experience (there are none in many areas). A good design will take more research, detailing, and follow-thru during construction than a barrier wall. Because of this attention to detail, not every project is a candidate for a rainscreen.