None of the rainwater striking a metal and glass curtain wall is absorbed, as it would be with masonry construction, and a substantial film of water will flow down the surface. If wind is present, the water may also flow laterally or even upwards on parts of the building: the taller the building, the greater will be the accumulated flow over the lower parts of its walls. Lateral flow under wind pressure is greatest near the windward corners of the building, and upward flow is
greatest near or at the top of the facade facing the wind. Lateral flow will also be concentrated at vertical irregularities in the wall surface, either projections or depressions, and these may often be joints. In general, the flow of water at vertical joints is much greater than the average flow of water over the wall.
A number of forces then act to move the surface water through any available opening. All of these forces are illustrated schematically in Fig. 3. Probably the most familiar of these is the force of gravity, and appropriate methods of counteracting this force are well known. Another force is kinetic energy: raindrops may approach the wall's surface with considerable velocity, and their momentum may carry them through any opening of sufficient size. Cover battens, splines or internal baffles can be used to prevent rain entry due to this type of force. A third factor that contributes to leakage is surface tension, which gives the water the ability to cling to and flow along soffit areas.The preventive to this action is in the form of a drip at the outer edge of the overhang. Fourth, capillary action is likely to occur whenever the space separating two wettable surfaces is small.The way to control water flow by capillarity is to introduce a discontinuity or air gap in the joint that is wider than the capillary path.
It is the next two forces caused by wind action that are the most critical, and the most difficult to combat. Air currents may result from differences in wind pressure over the wall surface, or from convection within wall cavities. These may carry water into the wall. Also, if water is present on one side of an opening and the air pressure on that side is greater than that on the other side, the water will be moved through the opening, no matter how small, in the direction of the pressure drop.This pressure difference may be caused by even the gentlest of winds and cause most of the leakage at wall joints.
The conventional approach to combating the last two forces was to attempt to eliminate all openings by using a tight seal, but the more effective and reliable approach is to eliminate the pressure differential across the opening. It is this approach that is known as the rain screen principle. The essential features of the rain screen and pressure-equalized wall construction are shown in Fig. 4. Sketch (a) indicates how, with the larger pressure on the outside, water is normally drawn in through the joint. Sketch (b) shows the condition where the pressures on two sides of the outer surface are made equal, thus preventing leakage by gravity, kinetic energy, surface tension or capillary action. Sketch (c) shows how, in order to withstand the effects of air currents and wind pressure, a continuous air space must be provided between the inner and outer skins of the construction.
The pressure-equalized wall consists essentially then of an outer skin (the rain screen) and an inner tight wall with an air space between the two. The pressure equalization is maintained by not tightly sealing the air space with the outside.The seals on the outer skin are known as the airseals (Figs 5, 6).
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