Walls leak when three conditions exist simultaneously: rainwater is on the wall; openings exist through which the rainwater can pass; forces are present to drive or draw the water inward. If any one of these conditions is eliminated, the wall won't leak. THE KEY: ELIMINATE FORCES To prevent rain penetration the forces that drive or draw the water into the wall must be lessened. Many forces act on a wall during a rainstorm: raindrop momentum; capillary suction; air currents; gravity; indoor/outdoor pressure differential. THE CULPRIT: INDOOR/OUTDOOR PRESSURE DIFFERENCE Raindrop momentum, capillary suction, wind, and gravity all can contribute to leaks, but the main cause is a drop in pressure from the outside to the inside of the building. To reach equilibrium, air moves from the area of high pressure (outdoors) to the area of low pressure (indoors). Outdoor air infiltrates the wall and carries rainwater with it. THE SOLUTION: THE RAIN SCREEN WALL A rain screen wall is a cavity wall with vent openings to the outside. The vents keep the air pressure in the cavity nearly as high as the outdoor air pressure. This equalizes the pressure on both sides of the exterior facing, or rain screen, and thereby eliminates the force that would draw air and water through the wall. THE BACKUP DEFENSE: THE DRAINAGE WALL A drainage wall acts as a secondary line of defense. It catches any water that leaks through and directs it out of the wall so it doesn't reach the interior of the building.