Christopher Matthews, a senior consultant and vice president at Glazing Consultants International, LLC, authored an excellent article providing an overview of the methods for managing water within wall systems.
Most buildings constructed today include a weather resistive barrier (WRB) as part of the wall system, as is typically required by building codes and good construction practice. This WRB is intended to protect the wall system components from water flowing to the interior of the exterior wall cladding materials by providing a drainage plane against which the water will flow to lower locations. Following this method to its logical conclusion should lead to a wall system which includes integrated flashing at locations below, to route the collected water out through the wall to the exterior. Regrettably, although the codes also require flashing as necessary to protect the walls, we often see this critical component omitted. This results in a system where the water flows down the WRB, but has no ready path to the exterior. With no planned drainage locations, the water collects within the wall system and eventually flows through the path of least resistance, usually draining to the interior or being absorbed to the interior through the WRB itself.