In planning practice for high performance buildings comparatively simple and reliable tools for the quantification of indoor climate conditions (temperature and humidity) are rare, whereas established tools (e.g. TRNSYS, ENERGY PLUS) are very comprehensive and require detailed user knowledge in operation. In the present paper, the development, implementation, verification, and exemplary application of a new thermal-hygric model for solving building-climatic challenging planning tasks is presented. The use of thermal insulation in buildings under hot climatic conditions, for energy saving in summer, has been verified in southern climates and the implementation into a user-friendly software, named CLIMT (Climate-Indoor-Moisture-Temperature) is shown. For the model calibration, a case study in Germany was chosen, in which interior and exterior temperature and relative humidity values were measured. Even though the building shows the peculiarity of an irregular user behavior, very good accordance between measured and calculated values could be achieved. In a further step, the application of the developed program CLIMT is demonstrated by means of a specific example: the hygrothermal optimization of a church building design, located in a hot humid climatic zone (Latakia, Syria). As part of this design, several challenging boundary conditions had to be considered. Besides specific building physical and building climatic requirements (high humidity and high temperatures), also structural requirements had to be taken into account. Furthermore, the new church building had to assimilate with the historically grown environment and the specific building typology traditions of the region. The optimized building design, facilitating a damage-free operation and low building energy consumption, is presented in detail and essential optimization steps are discussed. The represented model allows for a simplification of building climatic design, even in challenging southern climate conditions, through the implementation of a user-friendly software and is therefore particularly suitable for use in architectural planning offices.