Calculating Center-Glass Performance Indices of Windows

Abstract

Building envelope performance is strongly influenced by solar gain and heat transfer through windows. The majority of this energy gain or loss passes through the center-glass area of the glazing system. Various methods have been devised to calculate the corresponding center-glass performance indices. Solar heat gain coefficient (SHGC) and U-factor are the quantities most frequently sought. Hand calculations have given way to computer-based techniques. Computer simulation offers the opportunity to employ more detailed models plus the ability to model the large number of glazing systems made possible by design options, such as low-emissivity or solar-control coatings, selective glass tints, substitute fill gases, and glazing layers, that partially transmit longwave radiation. A new, more accurate method is presented in this paper for manipulating spectral optical data while calculating the energy related optical properties of glazing layers and glazing systems. The use of the same technique to track visible and ultraviolet radiation is also demonstrated. In addition, more refined methods are documented for calculating SHGC and U-factor while accounting for the thermal resistance of individual glazings.