Collins, Michael R.Wright, John L.2017-03-222017-03-222006QC-06-003 (RP-1311)http://hdl.handle.net/10012/11572©2006, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 112, Part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE’s prior written permission.The fenestration chapter of the 2005 ASHRAE Hand-book—Fundamentals (ASHRAE 2005) has long included methods for determining the U-factor and solar heat gain coef-ficient (SHGC), or window performance indices, using the radiative and convective heat transfer coefficients around a glazing layer. The present work examines the errors inherent in applying these standard calculation methods to window systems that include a single diathermanous layer (such as a shading layer), and new equations for calculating the perfor-mance indices are derived. Furthermore, the radiative heat transfer coefficients used in these calculations can be difficult to determine in the presence of a diathermanous layer. There-fore, a new and stable method of calculating radiative heat transfer coefficients is also presented. The effects of using the existing procedures are demonstrated using industry-standard software.enGlazingSolar heatingRadiative and convective heat transfer coefficientsFenestrationShadingDiathermanousEnergyU-factorArticle