The aim of this paper is to present a method and a tool based on cost-effectiveness analysis (CEA) for assessing energy efficiency improvements in buildings. The method is based on comparing costs of an energy efficiency improvement with its effects in terms of reduced energy use. A short review of the current practices in evaluating the economic efficiency and feasibility of energy efficiency measures is presented. A case example is presented with a calculation by using a CEA calculation method adapted for energy efficiency improvements in buildings. It is here concluded that the systematic appraisal of cost-effectiveness of energy efficiency investments would allow most economical projects to be implemented first, leading to greater overall economic efficiency.