Window design, especially glazing choices, is a critical factor for determining the effectiveness of passive solar design. Windows are like a knife has two side; one is useful and the other is harmful. In this paper the effects of windows' U-value, window orientation and windows size on annual heating and cooling energy demand is studied considering the both energy and investment costs. The study has been performed for three different climate zones; Amman, Aqaba and Berlin. Four types of windows have been studied; single glazed, double glazed L. double glazed H and triple glazed. The results show that heating load is highly sensitive to windows size and type as compared with cooling load. Also, it is shown that with a well-optimized glazed window energy saving can be reached up to 21%, 20% and 24% for Amman, Aqaba and Berlin, respectively. (C) 2011 Elsevier B.V. All rights reserved.