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Section detail of typical warm climate multi-component wall system.

exterior of the wall system (Figure 6). This layer is frequently a multipurpose material providing an air, water, and vapor barrier. Any breaks in this barrier will allow air and vapors to enter the cavity. The temperature of the metal studs can be below the dew point when adjacent to interior gypsum, and condensation will form, causing corrosion, along with insulation and gypsum board deterioration problems.

Moisture Control Designs for Moderate Zones

In moderate climate zones throughout the U.S., a "smart" vapor barrier is often installed in the wall system (Figure 7). It is typically located on the interior of the studs and has characteristics that allow variable amounts of air and water When installed correctly, the perm rating actually changes with the change of relative humidity. During the winter when RH is low, the perm rating is 1, and during the summer with high RH the perm rating changes to 10 (Figure 7, Centria #9).

Section detail of typical moderate climate multi component wall system.

4. Multi Component Wall Systems

Multi-component wall systems are made up of numerous individual components that require careful design attention in order to avoid some common challenges related to thermal, moisture, and structural concerns. Typical components and concerns include:

Metal Studs

A major structural concern with the use of metal studs on a concrete floor slab is the deflection in the floor that can be transferred to the exterior wall system. This design can cause the deflection stresses to pass through the studs to the single most expensive element of the wall, the outer wall material. The solution is to move the studs outboard of the floor slab and use slotted connectors to handle the deflection (Figure 8). Not only is this is a better design, it is also a less expensive solution.

Separating the metal studs from floor slab deflection.

Insulation

The second component is the thermal barrier, or insulation. Frequently fiberglass insulation is used in the cavity between the metal studs. While fiberglass has been tested to provide reliable thermal protection at a reasonable cost, there are concerns with its effectiveness when it is used in combination with metal studs, however. This has been documented in ASHRAE 90.1, which is the basis for many energy codes used in the U.S. (As of 2005, only 10 states do not reference this standard, while the other 40 states represent over 70 percent of the commercial construction in the U.S.) ASHRAE 90.1 requires reductions in the calculated value of the insulation by using

"correction factors" (Figure 9). Note that a six-inch stud, filled with fiberglass insulation, at 16-inch centers has a theoretical R-value of R-21 but is severely reduced to only a value of R-7.4. The reason for the reduction is based on the thermal conductivity of the metal studs and the net effect on the overall heat transfer of the wall system.

Nominal Framing Depth

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