942 Insulation Installed Overthe Purlin

One of the most economical methods of insulating the roof of a metal building, shown in Figure 9.14, is to stretch glass fiber blanket insulation over the purlins or trusses, prior to mounting the metal roof panels on top.

The roof purlin is a steel Z-shaped member with a flange width of approximately 2-1/2 inches. The standard purlin spacing for girts supporting roofs in the metal building industry is 5 feet. Using such a method, the insulation is compressed at the purlin/ panel interface, resulting in a thermal short circuit. The insulation thickness averages 2-4 inches, but can range up to 6 inches. While thicker blanket insulation can be specified to mitigate this, the thickness is limited by structural considerations. With the reduced insulation thickness at the purlin, there is a diminishing return on the investment in additional insulation. Table 9.7 is a summary of the installed R-values that might be expected using this installation technique.

Figure 9.13

Table 9.6 Roof insulation installed between metal trusses.5

Figure 9.14

Where:

Table 9.6 Roof insulation installed between metal trusses.5

Where:

Unbridged

R-Value

R-Value

R-Value

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