European directive on energy performance of buildings (2010) and related national technical rules force reduction of energy consumption of both new and reconstructed buildings due to the so-called passive standard. Consequently the thermal design of such buildings, utilizing advanced materials, structures and technologies, requires proper analysis of relevant physical processes, unlike classical evaluations of thermal resistance from one-dimensional stationary heat conduction. The paper demonstrates a possibility of compromise between complicated multi-physical models and realistic thermal estimation of buildings, as well as some optimization procedures in building design leading to energy reduction.