COLUMN BUILDING SCIENCES FIGURE 1 Gravity. A driving force for rain entry. FIGURE 2 Surface Tension. A driving force for rain entry. Water Film Kerf Water Film Flashing With Drip Edge Rainwater Can Flow Down Surfaces and Enter Through Openings and Cavities Rainwater Can Flow Around A Surface as a Result of Surface Tension Flashings Direct Gravity Flow Rainwater Back Toward the Exterior FIGURE 3 Capillarity. A driving force for rain entry. Providing a Kerf or Drip Edge Will Promote the Formation of a Water Droplet and Interrupt Flow FIGURE 4 Momentum. A driving force for rain entry. Cavity Water Film Water Film Outside Inside Capillary Suction Draws Water into Porous Material And Tiny Cracks Outside Inside Rain Droplets Can Be Carried Through a Wall By Their Own Momentum Cavity Acts as Capillary Break And Receptor for Capillary Water Interrupting Flow * Layer building materials to control momentum; and * Provide an air control layer to control air pressure . Figure 1 illustrates how rainwater entry from gravity (hydrostatic pressure) can be controlled by providing drainage and flashings. Figure 2 illustrates how rainwater entry from surface tension can be controlled by providing a kerf or drip edge. Figure 3 illustrates how rainwater entry from capillarity can be controlled by providing a capillary break such as a cavity. Figure 4 illustrates how rainwater entry from momentum can be controlled by designing a wall with no straight-through openings or by layering building materials. Figure 5 illustrates how rainwater entry from air pressure differences can be controlled by providing airtightness or an air control layer. Rain Entry By Momentum Can Be Prevented By Designing Wall Systems With No Straight Through Openings FIGURE 5 Air Pressure. A driving force for rain entry. Exterior Air Pressure (Pe) Cavity Air Pressure (Pc) Interior Air Pressure (Pi) High Pressure Low Pressure Pe>Pc>Pi Driven By Air Pressure Differences, Rain Droplets are Drawn Through Wall Openings from the Exterior to the Interior Exterior Air Cavity Air Pressure Pressure (Pe) (Pc) High Pressure Air Control Layer Interior Air Pressure (Pi) Low Pressure Pe=Pc>Pi The Air Control Layer on the Masonry Wall Reduces the Air Pressure Driving Force Across the Brick