935Metal Elements in Metal Building Walls

Many metal building walls are constructed from a corrugated sheet steel exterior skin, a layer of insulation, and sometimes a sheet steel V-rib inner liner. The inner liner can be fastened directly to the steel frame of the building. The exterior cladding is attached to the inner liner and the structural steel of the building through cold formed sheet steel elements called Z-girts. Fiber glass batt insulation is sandwiched between the inner liner and the exterior cladding. Figures 9.8 and 9.9 show details of a typical sheet steel wall.

The steel framing and metal siding materials provide additional opportunities for thermal short circuits to occur. The highly conductive path created by the metal in the girts, purlins and frames connected directly to the metal siding can result in even greater thermal short-cir-

Figure 9.8 Vertical section view.

Inner Liner

Figure 9.9 Horizontal section view.

Warm Side

Inner Liner

Figure 9.9 Horizontal section view.

cuiting than that discussed in Section 9.3.4 for metal studs in insulated walls. Because of this unobstructed high conductivity path, the temperature of the metal element is nearly the same through the entire assembly, rather than varying as heat flows through the assembly.

In laboratory tests, the introduction of 16 gauge Z-Girts spaced 8 feet apart reduced the R-value of a wall with no girts from 23.4 to 16.5 hr-ft2-°F/Btu.4 Substitution of Z-Girts made of 12-gauge steel reduced the R-val-ue even further, to 15.2 hr-ft2-°F/Btu. The extent of this heat loss will be dictated by the spacing of the Z-girts, the gauge of metal used and the contact between the metal girts and the metal wall panels.

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