Why might a metal building with R-19 insulation not meet the energy code?

Most energy codes with prescriptive compliance options state that each building component must meet the minimum values specified and that the insulation cannot be compressed. Metal buildings are typically insulated by draping batt-type insulation over the structural supports and attaching the metal panels, compressing the insulation at the supports (see Figure 1). The fasteners or connectors provide a "thermal short circuit" (known as thermal bridging) through the compressed insulation, significantly reducing the overall thermal performance and thus the full R-value of the insulation. For example, R-19 insulation installed using this technique yields an effective R-value of approximately R-11.

Figure 1

Source: http://www.energycodes.gov/support/metal_faq.stm