This paper presents quantitative results on the economics of different levels of thermal insulation for a building envelope. Results are calculated on the basis of an example building in Germany, which is a recently built single family house with materials commonly used in light-weight constructions. Defining the cost efficiency of an energy efficient building allows one to identify solutions which are already economically viable as well as to determine specific costs of the investment in an advanced sustainable building. (C) 2003 Elsevier Ltd. All rights reserved.