The European Union (EU) and its members states have implemented various programmes to prevent contamination of air by green-house gases according to comittments of the 2005 Kyoto Protocol. From 2008 to 2012 the countries having signed the Protocol, Slovenia being one of them, shall have to assure 5 % decrease in the green-house gases emissions compared to the state in 1990. To achieve the objective the EU adopted two directives. The first directive (SAVE 93/76/EEC) foresees reduction in the CO2 emissions through improvement of energy efficiency. The second directive (2002/91/EC) determines criteria for buildings energy efficiency. To implement the two directives Slovenia has adopted compulsory certification of buildings, on the basis of which energy-efficiency certificates will be granted in compliance with the common practice of some EU countries. The content and calculation methodology of the energy-efficiency certificate is based on the European standard EN 832. Besides the main pollutants, such as industry and traffic, residential and other buildings contribute a substantial share to pollution too. Pollution can be reduced by increasing the energy efficiency of buildings, realising during the heating season heat losses into the environment. As heat flows from the warmer interior into the colder exterior, heating the interior by different energy supplies is required in order to maintain constant temperature. Heat flows as conduction, convection and radiation into all directions depending on heat resistance of "obstacles". The paper presents a comparison of energy efficiency of two residental buildings. It is based on results obtained by a software developed at the LMK. The calculation is first made according to recommendations of the above standard and then on physical equations adapted to the used model of residental buildings. The software was developed in the Microsoft Excel environment in modules, calculating different characteristics of buildings and comparing two different buildings. The modules are the following: calculation of dimensions, windows and doors, temperature, input data, heat gains, heat losses - built house, losses - prefabricated house, comparison of results. Characteristic data of the used materials are obtained from the actual manufacturers. As the final results, heat losses for a residental building are given.
