In the last decades improving the solar control performance of the window has become of considerable importance to the designers and occupiers of large modern buildings. Window performance is reflected in building costs, through the importance of air-conditioning services. The options open to the designer are many and the quantification of the performance of the various window designs has become of primary importance. Two of the major areas of endeavor have been the window glass itself and the multitude of types of shading materials combined in the window to minimize the effect of solar heat gain. Several papers presented at previous ASHRAE meetings are referred to in the bibliography, in addition to the excellent information given in the ASHRAE Guide and Data book. This paper seeks to complement the major part of these data in offering procedures which will be of assistance to the designer.

One of the major problems facing the designer is lack of technical information on the solar optical properties of various manufacturers' materials. Often this is not available or too confusing for practical use. This problem has long been recognized and ASHBAE established a classification of drapery fabrics from which the designer could obtain data. In the meantime fabrics have begun to be used in louver systems, thus compounding the problem, and in any case the original fabric classifications, requiring judgments on the degree of openness and color, are subjective.

It was thus necessary to attempt to simplify an already complex problem which is threatened further by a widening range of solar control glasses.