A principal component of annual energy consumption in a building is attributable to energy transfers in the fenestration system. Annual energy requirements are not only a function of glazing properties, but also of other building design characteristics, operating characteristics, site conditions, and climate. This paper describes results of a study in which annual energy consumption with and without daylighting utilization in an office building module was modeled parametrically for a wide range of glazing properties in three different climates. We present results which suggest optimal combinations of glazing properties which frequently result in lower energy consumption than opaque insulated walls.