INTRODUCTION The ASHRAE Standards 90.1-1999 (ASHRAE 1999)@ 90.1-2001 (ASHRAE 2001)@ 90.1-2004 (ASHRAE 2004) and 90.1-2007 (ASHRAE 2007) contain appendices which list the installed U-factors for fiberglass batt insulation in metal building roof and wall assemblies. These U-factors were originally developed through finite element analysis (FEA) models that were validated against calibrated hot box (CHB) measurements (Graber 1998). Each U-factor was based on many assumptions that were considered to be typical or representative values at that time. The FEA models replicated the drape that was observed in the CHB tests. However@ the fundamental question was whether the CHB drape was representative of typical field installations and this has generated considerable debate. The installation and subsequent centerline drape of the fiberglass batts in metal building roof and wall assemblies are controlled by the insulation installer. Installation instructions exist but there are no specific criteria regarding the centerline drape necessary to achieve the desired thermal performance (NAIMA 2006). Thus@ field measurements of actual installations provide the best record as to the actual drape that exists. Once the actual drape is known then it can be used to determine the corresponding U-factor. Historically@ the U-factors were determined by either CHB tests or FEA modeling@ and both are expensive and time consuming. The SSPC 90.1 Envelope Subcommittee identified a need to be able to calculate these U-factors using simpler mathematical models. To develop these models they formed a task group to address these problems which were divided into four separate activities. The first activity was to develop new computational fluid dynamic (CFD) models which were then validated against CHB test results (Choudhary et al. 2010). Second@ a simplified model for a single layer of fiberglass batts was developed and correlated back to the CFD results (Choudhary and Kasprzak 2010). Third was the development of a simple model for double layers of fiberglass batts@ which is the focus of this paper. Fourth@ field and experimental laboratory measurements of typical or representative drape profiles@ including center line measurements@ were completed and those results were used in combination with the simplified models to calculate the proposed roof and wall assembly Ufactors for Standard 90.1-2010 (Christianson 2010). Double layers of fiberglass batt insulation are a typical construction option used to achieve higher thermal performance in metal building roofs and walls. When the double layers have different material properties it is more difficult to determine the combined U-factor. To address this fundamental problem a series of experimental tests were completed and the results were used to develop a simplified U-factor calculation procedure.