Many exterior walls have air gaps or drainage systems in their design, to deal with moisture intrusion.
Behind these systems is the assumption that moisture migration into exterior walls is unavoidable, and that the best defense relies on such drainage and drying systems.
Rainscreen walls (Wood frame walls)
Wood frame walls need a rainscreen - unless you live in a very dry climate, or the wall is especially protected by a large roof overhang or a porch.
Rainscreens are critical in rainy climates.
We shouldn't forget that modern exterior walls tend to be airtight assemblies, and that wood frame construction is very exposed to moisture damage.
Rainscreen walls comprise an air gap between the back of the siding and the a Water Resistive Barrier (WRB), and should be comprehensively flashed and have weep holes at its bottom.
Obvisouly, that isn't enough to keep wood-frame walls dry.
This involves also: roof overhangs and a well-designed ground drainage system (what we may call the first line of defense); the use of moisture resistant materials (air barriers; water resistive barriers, vapor barriers); capillary breaks, to avoid upward moisture suction; comprehensive flashing at all wall penetrations and junctures (flashing is especially important); proper siding and wall coverings.
All these features should be implemented with great attention to detail. Bad flashing, unsuitable overhangs or poor designed ground drainage systems will compromise the goal of keeping the walls dry.
Drainage Cavities in Masonry walls
Though masonry walls are more resistant to water damage than wood frame walls, they too can be damaged by water; on the other hand, keeping masonry walls dry is not easy.
Traditional masonry exterior walls are of cavity type - they include a cavity/air space of 1-2 inches/25-50 mm wide to stop moisture infiltration, and also a drainage plane covered with a WBS, that is, a weather resistive barrier (to drain water to the bottom of the wall cavity).
The insulation layer of the wall (typically polystyrene or mineral wool….) and the air/water resistant barriers are generally located between the air space of the wall and the inner wythe.
Details are critical, especially flashing and associated weeps.
The cavity should be properly ventilated (with venting weeps and cavity vents); ventilation will avoid moisture suction and problems associated to air presssure differentials in the cavity, caused by wind intrusion and the building’s HAVC systems.
Note: solid masonry walls, with a fully-filled (with insulation) cavity, are now increasingly common. But there are risks in this approach. Without a drainage cavity and water repellent materials (or a drainage mat over the insulation material) the risk of moisture damage in masonry walls becomes high.