The effect of slat angle and absorptance on the heating and cooling loads of buildings in Seoul (South Korea) has been studied. An analytical model of a window system with venetian blinds is developed and validated with published experimental data. The validated model has been successfully incorporated into the TRNSYS computer program. Hour-by-hour heating/cooling loads of a typical building, with and without venetian blinds, are compared. Results show that the use of windows with venetian blinds can reduce the building heating load by about 5% and cooling loads by as much as 30%.