Vapor-Permeable Air and Water Barrier In Cold Climates
Applied over the sheathing is a vapor-permeable air and water barrier or retarder, usually made of woven manufactured product. As a moisture retarder, it allows vapors to penetrate the interior stud space cavity to pass through into the outer drainage cavity. At the same time, it acts as a barrier to both air and liquid water coming from the outside. Hence, it must be fastened properly to withstand the imposed air or water that it may be subjected to but also provide continuity to maintain its integrity and effectiveness. Critical to good performance, all joints must be properly taped and secured with particular attention to detail at the window and door heads, sills, and jambs.

Drainage Cavity
The outer material is often referred to as a rainscreen, particularly if it is a metal material that is not intended to be airtight and is primarily intended to be the weathering surface that screens the rest of the wall system from rain and other elements. It may be attached in various ways that penetrate through the air and water barrier and gypsum sheathing into the studs. The space created between the rainscreen and the barrier-covered sheathing is called the drainage cavity. It must have enough space for moisture to properly drain out and still accomplish air pressure equalization, which is important in windy conditions. The drainage cavity is sometimes divided into compartments that help maintain pressure equalization.

5. Rainscreens

The term rainscreens refers to the outer material in a wall system, usually when it is made out of metal cladding. There are several different metal cladding types:

Single Skin or Roll-Formed units.
This common type of metal cladding has very desirable design characteristics. These materials provide variations in texture from light striations to bold ribs, which can provide light or bold textures. They can also be installed to run horizontally or vertically.

Foam Insulated units.
Metal cladding may also be an insulated composite (Figure 10) . Design characteristics of this metal cladding type include high-performance pressure equalized joinery, and the ability of the units to be curved to various radii. They also can be installed in horizontal or vertical applications, using both wide horizontal reveals and wide vertical reveals recessed to the same plane as a standard detail. They integrate easily with glass curtainwall and windows.

Figure 10 Examples of foam insulated metal cladding units.

Thin Composite Metal Units
Thin metal composite walls consist of two layers of metal, with a composite core that is either formed or fabricated into a panel system. Some joinery systems have geometry that is a pressure-equalized rainscreen. These composite walls are created for superior flatness, wide modules, and grid look. Design characteristics include crisp sight lines, smooth curves, and a high-tech grid look.

Plate Systems
Plate systems are available in many types of designs. They generally are used on medium or high-rise construction.