We often clad walls with large pieces of less porous materials such as panels of granite or dense pre-cast concrete, or sheets of metal or glass.
With these materials, the problem is not one of stopping an overall slow seepage, as it's with masonry walls, but of preventing leakage through the joints between pieces. Such large panels can't be filled tightly together because of their relatively large amounts of thermal expansion and contraction and because of the unavoidable inaccuracies in their manufacture and installation. Quarter-inch to three-quarter- inch (6 mm to 9 mm) gaps must be provided on all sides of each panel, and these gaps must be made resistant to the passage of water.
The easiest way to make these gaps watertight, though not the most reliable, is to fill each gap with an airtight, watertight sealant, a liquid synthetic rubber that is injected against a foam plastic backer rod and hardens into a continuous elastic plug that adheres to both sides of the gap. Alternatively, a molded synthetic rubber gasket can be tucked into the gap (ill. 10-12). In theory, the sealant or gasket expands and contracts readily with any movement of the panels, all the while maintaining its tight seal against wind and water. In practice, even the best sealant or gasket joints will deteriorate over a period of years, particularly if they are exposed to the 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.
ill. 10-12: Sealant and Gasket Joints
More reliable joints between wall panels are produced by reduc ing or eliminating the joint’s dependence on a sealant material for its water resistance. Going back to our theory of watertightness, 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's 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 can't be thrown through
the joint without striking a surface that blocks its passage (ill. 11-12). Notice that the baffles in a labyrinth don't touch one another. Rather, they are spaced far enough apart that a drop of water can't bridge between them, thus preventing capillary entry as well as kinetic entry. it's 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.
ill. 11-12: Labyrinth Joints
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 (ill. 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.
ill. 12-12
The same effect can be achieved without the use of sealant by providing a continuous layer of air behind a facade of unsealed, labyrinth-jointed exterior panels to create a wall that works by the rainscreen principle. The backup wall behind the airspace must be airtight, must be sealed to the panels around the edges of the facade, and must be strong enough structurally to resist the expected wind pressures on the building. Very small amounts of air passing back and forth through the joints serve to maintain the air pressure in the airspace at exactly the same level as the wind pressure outside at any instant. This effectively eliminates any air-induced movement of water through the joints. In this arrangement, the panels form a rainscreen to divert raindrops. The backup wall forms an air barrier. The space between is a pressure equalization chamber (PEC) that prevents the formation of air pressure differentials between the inside and outside of the joints in the rainscreen (ill. 13-12). The backup wall can be made very simply—of concrete blocks surfaced with an airtight mastic, for example, or of light wood or steel framing with sheathing panels covered by an air barrier of asphalt-impregnated building paper cemented together at the seams.
ill. 13-12: Principle the Rainscreen: Wind pressure P1=P2, pressure
in PEC. Therefore wind can't force water through joints in rainscreen.
In the example of rainscreen cladding shown here (ill. 14-12), a multistory building is faced with panels of thick sheet aluminum. No joint sealant is used. The open joints between the panels are configured so that they systematically neutralize all the forces that might move water through them. Gravity is defeated by sloping the horizontal surfaces of the joints toward the outdoors. Capillary action is eliminated by keeping the panels a sufficient distance apart at the joints. Labyrinth joints keep out water that is carried by momentum. Air pressure differentials are neutralized by the pressure equalization chamber behind the panels. The PEC is divided into compartments by the horizontal aluminum components at each floor line and by the aluminum ribs that support the vertical edges of the panels. This compartmentation is desirable because wind pressures vary considerably across the face of a building, particularly near the corners and the top edge. If the PEC were not compartmented, air could rush through it from an area of higher pressure to one of lower pressure, drawing water through the joints in the higher-pressure area. The PEC should be allowed to drain freely at the bottom to rid itself of any water that might somehow get past the rainscreen.
ill. 14-12: Building With Rainscreen Wall.
The rainscreen principle can be applied effectively even to very small exterior details such as door and window frames (ill. 15-12). The exterior sill should have a sloping top surface, or wash, to divert water away from the door or sash, and it should project over the wall below, with a groove called a drip to prevent clinging water from working its way back into the wall beneath the sill. A capillary break should be provided where the sash meets the sill, to keep water from being drawn through the crack by capillary action. If the sash is weatherstripped on the interior side of the capillary break, the capillary break will also serve as a pressure equalization chamber, making it very difficult for wind to drive water through the crack. The weatherstripping is the air barrier that maintains pressure in the PEC.
ill. 15-12: Window Sill Profile: a. Drip; b. Capillary break; c. Weatherstripping
(air barrier); d. Wash
Automobile makers utilize rainscreen detailing routinely around the edges of doors, trunk lids, and hatches (ill. 16-12). Gaskets are used as air barriers and are placed well inside the large gap between the door and the body of the car, behind a PEC.
ill. 16-12
With horizontal wood siding or shingles, horizontal overlapping is sufficient to make the wall watertight except under unusually severe conditions of wind-driven water. The siding or shingles should be backed up by a barrier to the passage of air such as a sheet of asphalt- impregnated felt or bonded plastic fibers. For best performance, the siding or shingles should be spaced out, away from the air barrier, with strips of wood (ill. 17-12). When configured in this way, the siding becomes a rainscreen and the space is a PEC.
ill. 17-12
There have been many water leakage problems in the wetter climates of North America with a siding system called exterior insulation and finish system (EIFS). EIFS consists of a layer of rigid plastic foam insulating boards covered with a very thin coat of synthetic stucco that is reinforced with a glass fiber mesh. In the buildings that have experienced the worst problems, the insulating boards are adhered directly to the air barrier layer that is attached to the wall (ill. 18-12). Leakage can occur either because of damage to the thin stucco layer or because of poor workmanship in making sealant joints where the stucco joins window and door frames. This leakage would be of relatively little consequence if there were a drained airspace behind the insulating boards, because then the system would function essentially as a rainscreen wall (ill. 19-12). However, when the insulating boards are attached directly to the wall of the building, there is no place for moisture to go once it has leaked in. It remains in the wall, causing rot, rust, mildew, and mold. Thousands of houses have had to have entire walls removed and replaced, and repair costs total tens of millions of dollars.
ill. 18-12 (left) and 19-12 (right): Wrong and right way to implement
a rainscreen. Loosley-woven plastic mat. Forms PEC and drainage layer.
Splashing mud from the drip line of a roof poses only a problem of unsightliness in the case of concrete or masonry walls, but soil microorganisms can cause severe decay problems in wood siding. A wise precaution is never to allow wood to be closer than six inches (150 mm) to the ground. A clearance of eight inches to a foot (200 to 300 mm) is even better. Moisture from the soil may rise up through foundation walls of masonry or concrete by capillary action, to cause dampness and decay in ground floor rooms. A flashing through the wall just above the ground line cures this problem.
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