Calculating Window Solar Heat Gain

Abstract

Window design has been revolutionized-largely by the introduction of low emissivity (low-e) coatings and substitute fill gases. The large number of design options necessitates the use of computer simulation for development and rating. Two window analysis programs, VISION1 and WINDOW,2 are widely used in North America. Both have been released in several versions-the most recent being WINDOW 4.1 and VISION3. They differ in appearance because WINDOW is text based and VISION3 incorporates a graphical user interface (GUI) but they perform similar solar optical and heat transfer calculations to arrive at center-glass U-factors and solar heat gain values. More detail can be found in Wright's "Summary and comparison of methods to calculate Solar heat gain".

This article examines window solar heat gain-how it is calculated and what affects it. Solar heat gain is quantified by the Solar Heat Gain Coefficient (SHGC). SHGC is the fraction of inci­dent solar radiation that reaches the conditioned space. It is customary to consider each of three areas: (1) the center-glass area, A, (i.e., the glazed area more than 2.5 inches (63.5 mm) from any sight line, (2) the edge-glass area, and (3) the frame area, Component SHGC values are area-weighted to give a total window SHGC.