Deficiencies in systematic approaches for design decision support to increase energy and environmental performance of buildings in Turkey are projected as the major problem of this study. Rare usage of computational methods for evaluating and improving building performance, need for informational assistance in design, as well as lack of interaction between systematic knowledge and building practice are the secondary problems. This thesis mainly aims to test usability of the decision support process assisting architects in early design stages of residential buildings. Assessing the uncertainties in building performance caused by design parameters and climate change, in terms of determining the most significant parameters on annual energy consumption for heating, cooling and operational CO2 emissions in hot humid climatic region of Turkey is the significant objective of the study. The relation between input parameters and building performance indicators is examined by the uncertainty and global sensitivity analyses for a residential building in İzmir, Turkey. The process is supported by usability testing held with the architect in practice. The results indicate that, the sensitivity between input and output parameters changes according to the projected weather conditions and different floors of the building. The SHGC of windows on south-west, north-east facades, and window U values are the most effective parameters on energy and environmental performance. The proposed decision support process is approved to be applicable for early design stages of the selected building, and helps creating consciousness about the importance of systematic design decision approach for the building professional.