Metal Building Roofs

Many of the strategies suggested for metal walls are applicable to metal roofs. In addition to strategies that minimize thermal bridging due to metal elements, other features of metal roof construction can have a significant impact on the thermal performance.

Tests have shown significant variation in the thermal performance of insulation depending upon the facing used, even though the permeability of the facing itself has no significant thermal effect. The cause of this difference

Vinyl Facing Figure 9.15 Vinyl facing.

Figure 9.16 Vinyl facing with glass fiber scrim.

9.4.3.1 Use the "Roll-Runner" Method of

Installing "Over-the-Purlin" Insulation The "Roll-Runner" installation technique helps to mitigate the effect described above, achieving less insulation compression by improving the draping characteristics of the insulation. This is done using bands or straps to support the suspended batt insulation, as illustrated in Figure 9.17.

Figure 9.17 Roll Runner Method

Figure 9.18 Insulated purlin with "roll-runner" installation.

Figure 9.16 Vinyl facing with glass fiber scrim.

Figure 9.18 Insulated purlin with "roll-runner" installation.

9.4.3.2 Use Thermal Spacers Between Purlin and Standing Seam Roof Deck

Figure 9.18 depicts a thermal spacer installed between the top of the purlin and the metal roof deck. The impact of this strategy will vary with the geometry of the individual component as well as the location of the spacer.

Table 9.8 summarizes the comparative performance of the two installation techniques discussed above. In all cases, purlins were installed 5 feet on-center. While improvements of 8 to 19 percent in effective R-value are achieved using the Roll-Runner method, performance of 60 to 80 percent can be realized when the purlins were also insulated from direct contact with the metal structure.

9.4.3.3 Install Additional Uncompressed Insulation Between Purlins or Bar Joists

This insulation system illustrated in Figure 9.19, is sometimes referred to as "Full Depth/Sealed Cavity," referring to the additional layer of insulation that is installed between purlins or bar joists, and which is not compressed as is the over-the-purlin insulation above it. In this configuration, the main function of the over-the-purlin insulation above is to act as a thermal break.

9.4.3.4 Add Rigid Insulation Outside of Purlins

The greatest benefit will be derived where insulation can be added which is neither compressed nor penetrated by conductive elements, as shown in Figure 9.20.

Table 9.9 comparatively summarizes the performance of the above installation for varying levels of insulation installed over purlins with varying on-center spacing.

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