The paper extends the application of a correlation-basedapproach developed earlier and adopted by the AHRAE StandingStandard Project Committee (SSPC) 90.1 for calculatingthe overall heat transfer coefficients (the U-factors) of singleand double layered fiberglass metal building (MB) insulationassemblies to assemblies where insulation is used in a mannerdesigned to fill up the space (the cavity) between the twoconsecutive structural elements (purlins or girts). Theseassemblies may involve either single or multiple insulationlayers, and the cavity may be filled to various extents. Thepaper describes a general technical approach to calculate thethermal resistances of various regions of an MB insulationassembly, namely regions beyond, underneath, and above thestructural units. These zonal thermal resistances are combinedinto a single, overall or system thermal resistance, R insul-sysusing series and parallel combinations while also allowing forthe thermal short circuiting or preferential heat flow throughthe metallic structural elements and the thermal resistances ofthe ambient air. The correlation, referred to as the Choudharycorrelation, calculates the assembly U-factor toRinsul-sys usingthe following equation.1 ? U = 0.8672Rinsul-sys + 1.132

In the above equation U is in Btu/ft2·h·°F and R is inft2·h·°F/Btu (1 Btu/ft2·h·°F=5.6782W/m2·K and 1 ft2·h·°F/Btu= 0.17611 m2·K/W).The U-factors for 13MBinsulation assemblies, consistingof a wide range of fiberglass insulations calculated using thecorrelation, are found to be in 93.5% to 100% agreement withthe values calculated using three-dimensional numericalapproaches. In addition, for four assemblies for whichmeasured U-factors were available, the correlation basedresults were in 91% to 100% agreement with the measuredvalues. The Choudhary correlation is strictly applicable to afixed spacing of 5 ft (1.52 m) between two consecutive structuralelements.