The Kyoto protocol adopted in 1997 [1] was aimed at fight- ing the constant increase in worldwide energy consumption and, as a result, the increase in greenhouse gas emissions. The leading industrial countries committed to reduce their annual greenhouse gas emissions by an average of 5% for the period 2008-2010. As buildings account for more than one third of total energy consump- tion in the European Union and have an enormous energy-saving
potential, the EU directive [2] sets national binding targets for CO2
reductions. Saving energy in the building sector is therefore a cen-
tral key-point with regard to climate protection.