A simple method of comparing the thermal performance of glazing systems is presented. A universal performance characteristic can be established for each glazing system, using only its average over-all heat-transfer coefficient and shading coefficient. The method also permits the calculation of the net heat gain attainable with a given glazing system. The validity of the assumption of constant window thermal and optical properties is confirmed through comparison with values of net heat gain obtained by detailed calculations using actual hourly atmospheric data for four Canadian locations. The method is extended to permit prediction of the performance of windows using nighttime insulating shutters. An example of its use is given.