Paper undertakes software based numerical simulations in two different traditional houses in cold-dry climate of Tabriz and hot-arid climate of Yazd, The Eco Test software chosen for BIM simulations. Case study on real houses instead of hypothetical models can give more tangible results and choosing traditional houses can provide a good opportunity to compare both physical and thermal conditions, in which zones have more dense walls than contemporary typical building wall. With high thermal mass and more surface to volume ratio, investigating the effect of U-value on interior conditions are more accurate. Comparative case studies in two different climatic zones of Iran can give researchers the opportunity to compare the importance of total U-value in two severe climates in Iran. In addition, it can give the designers the insight that what design priorities must be considered in each climate. Results show that the U-value in a hot arid climate of Yazd has a more direct effect in controlling interior comfort conditions, like fluctuations of temperature and variations of summer and winter temperatures, as well as variations of temperature in a single day and total energy need and consumption in both hot months and cold months of the year.