A rainscreen is an exterior wall detail where the siding (wall cladding) stands off from the moisture-resistant surface of an air barrier applied to the sheathing (sheeting) to create a capillary brake and to allow drainage and evaporation. The rain screen is the siding itself but the term rainscreen implies a system of building. Ideally the rain screen prevents the wall air/moisture barrier on sheathing from getting wet. In some cases a rainscreen wall is called a pressure-equalized rainscreen wall where the ventilation openings are large enough for the air pressure to nearly equalize on both sides of the rain screen, but this name has been criticized as being redundant and is only be useful to scientists and engineers. For water to enter a wall first the water must get onto the wall and the wall must have openings. Water can then enter the wall by capillary action, gravity, momentum, and air pressure (wind). The rainscreen system provides for two lines of defense against the water intrusion into the walls: The rainscreen and a means to dissipate leakage often referred to as a channel. In a rainscreen the air gap allows the circulation of air on the moisture barrier. (These may or not also serve as a vapour barrier, which can be installed on the interior or exterior side of the insulation depending on the climate). This helps helps direct water away from the main exterior wall which in many climates is insulated. Keeping the insulation dry helps prevent problems such as mold formation and water leakage. The vapour-permeable air/weather barrier prevents water molecules from entering the insulated cavity but allows the passage of vapour, thus reducing the trapping of moisture within the main wall assembly. The air gap (or cavity) can be created in several ways. One method is to use furring (battens, strapping) fastened vertically to the wall. Ventilation openings are made at the bottom and top of the wall so air can naturally rise through the cavity. Wall penetrations including windows and doors require special care to maintain the ventilation. In the pressure-equalized system the ventilation openings must be large enough to allow air-flow to equalize the pressure on both sides of the cladding. A ratio of 10:1 cladding leakage area to ventilation area has been suggested. A water/air resistant membrane is placed between the furring and the sheathing to prevent rain water from entering the wall structure. The membrane directs water away and toward special drip edge flashings which protect other parts of the building. Insulation may be provided beneath the membrane. The thickness of insulation is determined by building code requirements as well as performance requirements set out by the architect. The system is a form of double-wall construction that uses an outer layer to keep out the rain and an inner layer to provide thermal insulation, prevent excessive air leakage and carry wind loading. The outer layer breathes like a skin while the inner layer reduces energy losses. The structural frame of the building is kept absolutely dry, as water never reaches it or the thermal insulation. Evaporation and drainage in the cavity removes water that penetrates between panel joints. Water droplets are not driven through the panel joints or openings because the rainscreen principle means that wind pressure acting on the outer face of the panel is equalized in the cavity. Therefore, there is no significant pressure differential to drive the rain through joints. During extreme weather, a minimal amount of water may penetrate the outer cladding. This, however, will run as droplets down the back of the cladding sheets and be dissipated through evaporation and drainage.