Under the Kyoto protocol, the European Union has committed itself to reduce the emissions of greenhouse gases (GHG) by 8 % in the period 2008 to 2012 compared to the level in the year 1990. The emissions of the most important greenhouse gas CO2 are mostly linked to the combustion of fossil energy carriers and therefore, increases in energy efficiency are expected to contribute considerably to the achievement of GHG emission reduction goals. In particular energy use in buildings accounts for more than 40 % of the final energy demand in the EU.