ANALYSIS OF THE THERMAL AND DAYLIGHTING PERFORMANCE OF SINGLE- AND DOUBLE-GLAZED WINDOWS OF VARIOUS SIZES IS USED TO ILLUSTRATE WAYS IN WHICH THE NATURAL LIGHTING AND SOLAR BENEFITS OF WINDOWS CAN PARTIALLY OR FULLY OFFSET THEIR NORMAL HEAT CONDUCTION DISADVANTAGES. THE EFFECTS OF VARYING THE OPERATION OF A WINDOW ARE STUDIED BY MODELING BOTH THE SELECTIVE MANAGEMENT OF INTERNAL WINDOW COVERINGS (FENESTRATION MANAGEMENT), AND THE SUBSTITUTION OF NATURAL LIGHT FOR ARTIFICIAL LIGHT (WHENEVER PRACTICABLE). THREE DIFFERENT MODES OF OPERATION FOR HEATING AND COOLING REQUIREMENTS THROUGH WINDOWS WERE ASSESSED: EXTERNAL LOADS ONLY; EXTERNAL AND INTERNAL; AND EXTERNAL, INTERNAL, AND DAYLIGHT. THE USE OF DAYLIGHT OFFERED THE GREATEST POTENTIAL FOR REDUCING ENERGY COSTS, PARTICULARLY WHEN COMBINED WITH SELECTIVE FENESTRATION MANAGEMENT. THE REDUCTION WAS GREATEST FOR MANAGED SOUTH-FACING, DOUBLE-GLAZED WINDOWS IN WHICH DAYLIGHT REPLACES OR SUPPLEMENTS ARTIFICIAL LIGHT. IN ALL CASES, HOWEVER, REDUCING OPERATING COSTS FOR WINDOWS REQUIRED CAREFUL CONSIDERATION OF ORIENTATION, USE OF DAYLIGHT WHENEVER SUFFICIENT, AND OFF-HOUR TEMPERATURE SETBACK. DIAGRAMS AND PHOTOGRAPHS ACCOMPANY THE TEXT. (AUTHOR ABSTRACT MODIFIED--MHP)