ASHRAE 90.1 correction factors for metal stud walls.
This component of the wall system is climate dependent with stud cavity insulation. In cold climates, it is located on the interior of the metal studs. A key to good performance of a vapor barrier-retarder is continuity of the material, so as to avoid gaps or breaches. Challenges in maintaining this continuity occur at floor slabs, behind spandrel beams, at electrical box penetrations, and at the roof and wall intersection at parapets. The condition at the parapet is critical because it relies on coordinating wall and roofing contractors to maintain vapor barrier continuity. Electrical distribution is a challenge because frequently it is done in the stud cavity. Hence, at every electrical box, the vapor barrier is penetrated, causing a breach in the vapor barrier continuity.
Typical problems with vapor barrier-retarder continuity.
Exterior grade gypsum sheathing is applied to the exterior side of the metal studs. There are advantages and challenges with this type of installation. Advantages include quick enclosure of the building, fire resistance, and the creation of a continuous surface to apply building wraps. Challenges associated with the gypsum sheathing material include the necessary penetrations to secure the outer material or cladding attachments, and the resulting exposure of the gypsum core to moisture.
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.
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.
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:
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.
Metal cladding may also be an insulated composite. 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.
Examples of foam insulated metal cladding 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 are available in many types of designs. They generally are used on medium or high-rise construction.
Custom Wall Systems
Custom wall systems are almost limitless in what can be accomplished.
When comparing the different types of rainscreens, it is important to remember that they serve several important functions. Beyond providing an outward appearance and wearing surface, they allow a wall system to drain liquid and to vent water vapor from external leakage, internal vapor diffusion or internal air leakage. In general, rainscreens allow ventilation behind the metal to help eliminate moisture in the drainage cavity. In a 1988 publication entitled, "Rainscreen Cladding, a Guide to Design Principles and Practice," by Anderson and Gill, two types of rainscreens are defined, backside ventilated, and pressure equalized.
When comparing the different types of rainscreens, it is important to remember that they serve several important functions.
According to the authors, "back-ventilated...claddings are allowed to leak, and no deliberate attempt is made to minimize the effects of wind by means of pressure equalization...large quantities of rainwater penetrate the joints and run down the reverse, hidden face of the cladding assembly." (Figure 12).
The second type of rainscreen is a pressure-equalized system. Joseph Lstiburek, Ph.D, P.Eng., principal of the Building Science Corporation, has studied wall systems in considerable depth. He describes the most effective wall system as one that prevents water from entering the wall cavity and wetting the inner layers, while
Examples of foam insulated metal cladding units.
figure 12 figure 12
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