Buildings consume half of all energy use and are also responsible for a similar proportion of carbon dioxide emission. The heat transfer across the building envelope - the shell of a house that separates the inside and outside - should generally be minimized. In the paper validation and verification based on Building Energy Simulation Test (BESTEST) of Energy3D computer code is presented. Next, computations performed by means of Energy 3D and Energy Plus for BESTEST building are compared. In the last part of the paper results for computations for real building are presented. Program Energy 3D proved to be an excellent tool for qualitative and quantitative analysis of buildings with respect to energy consumption.