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4 When trade-offs or substitutions are required the energy code compliance of the buildings envelope can be verified using COMcheck a free program available from the DOE at comcheck. Non-prescribed systems can be entered into COMcheck using the Other U-Factor Option and then the Metal Building Roof drop-down as shown below As guidance the following is a review of new High R-ValueLow U-Factor insulation systems currently available for metal building roofs and walls. Roof Insulation High R-Valuelow U-Factor metal building roof insulation systems can be broken down into 2 generic categories filled cavity fiber glass and rigid continu- ous foam insulation. Filled Cavity OptionsU-Factors of 0.037 or Less Filled cavity options with U-Factors or 0.037 or less include Long Tab Banded LTB system. Liner system. Both the LTB and liner systems fill the cavity between the purlins with fiber glass insulation. A minimum of 2 layers are used one layer is installed between and parallel to the purlins and one layer on top of and perpendicular to the purlins. In both cases the insulation is supported by a series of bands attached to the underside of the purlins. From a thermal performance standpoint both systems perform comparably. Contrary to some of the claims being made in the marketplace actual side- by-side thermal testing by an independent accredited laboratory has demonstrated comparable performance with both the LTB and liner systems. ASTM C1363 test results for 2 LTB and liner assemblies using 2 different insulation levels are shown in Table 1. These results demonstrate that the filled cavity LTB system and the liner system performed comparably in this side-by-side test program. Due to the inherent variability in this type of testing the slight difference seen should not be considered to be significant. The primary differences between these 2 systems relate to the orientation of the vapor retarder facings With the LTB system the vapor retarder is laminated to the lower fiber glass layer and the facing tabs follow along and over the purlins where they are sealed together to maintain the continuity of the vapor retarder. With the LTB system the lower purlin flanges are exposed which allows for unobstructed instal-