12.11
weather, and will ultimately crack or lose their grip on the panels. Joints sealed with anything but the finest workmanship will fail much sooner; often they leak from the moment they are installed. More reliable joints between wall panels are produced by reducing or eliminating the joint's dependence on a sealant material for its water resistance. Going back to our theory of watertighttness, this means that either water must be kept away from the joint or the forces that can move water through the joint must be neutralized. It is nearly impossible to keep water completely away from joints in walls. Thus we must turn our attention to neutralizing the forces that might move the water through such joints. The three primary forces that move water through wall joints are the momentum of impinging raindrops, capillary action, and differential air pressures. The momentum of raindrops can be effectively stopped by means of a simple labyrinth consisting of interleaving baffles arranged in such a way that a drop cannot be thrown through the joint without striking a surface that blocks its passage (12.11). Notice that the baffles in a labyrinth do not touch one another. Rather, they are spaced far enough apart that a drop of water cannot bridge between them, thus preventing capillary entry as well as kinetic entry. It is important that labyrinth joints drain freely, to get rid of the water that they normally trap in the performance of their function. Corners and intersections of panels must be designed especially carefully, to drain the vertical joints without flooding the horizontal ones. Air pressure differentials are created wherever wind strikes a building surface. The windward exterior face of a building is often at a higher air pressure than are the rooms just behind that face. Under this condition, water present in a joint in the wall, even a labyrinth joint, can be carried into the building by the force of moving air. In order to stop the movement of air through the joint, we could apply a sealant to the exterior edges of the joint, but any imperfection in the sealant would allow a stream of moving air to carry water into the building (12.12). If instead we apply the sealant to the interior edges of the joint, the sealant will be exposed to air but not to water, which is excluded by the labyrinth. Even if the sealant is defective, for example, if it does not adhere perfectly to one side of the joint, only small volumes of air are likely to pass, not enough to transport water through the labyrinth. Furthermore, the sealant joint in this case is protected from the deteriorating effects of sunlight and water and, if suitably detailed, is accessible from inside the building for inspection and maintenance. A reliably waterproof joint is thus achieved with a minimum of means. The same effect can be achieved without the use of sealant by providing a continuous layer of air behind a facade of unsealed,
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