At present, most products and processes are optimised according only to their economic performance and disregarding environmental aspects. To promote a more sustainable economy, however, the environmental performance should be accounted for in the analysis. The prevalent method to include the environmental impact as a key aspect in decision-making relies on the use of multi-objective optimisation. Following this approach, the environmental and the economic performance are quantified separately as two different objectives, and the final result is given by a set of Pareto optimal solutions. In this study, we resort to eco-costs, a method that translates the environmental impact of a product or activity into monetary units, which can then be incorporated explicitly into the economic performance assessment. Hence, a unique optimal solution is attained, thereby avoiding the task of deciding among different optimal alternatives. The approach presented is illustrated through a case study where we test the eco-costs capabilities in the building sector. The objective is to optimise the thermal insulation of a building envelope in different climate zones. Our approach identifies building solutions that improve significantly the environmental performance at a marginal increase in cost.