||The theme of buildings' energy efficiency occupies a prominent place on the agenda. This study aims to assess the convenience of improving the energy performance of residential buildings: higher costs to be incurred during construction or renovation are offset by lower operating costs? The point of view is purely financial, externalities are not considered. The evaluation model is set on the discounted cash flow analysis, and partly based on life cycle assessment methodology. Variables are simplified by comparing the incremental costs of investment with the savings achieved on energy supply cost.The model is applied to three case studies located in the Northern Italy. The first is related to the design of a new single-family house. The second and third cases concern the renovation of two different public housing estates. For all cases several improvement alternatives are analyzed.The estimate of the investments constitutes a critical element; the literature has shown that many thermo-economic studies may underestimate them, due to the lack of consideration of hidden costs. The discount rate is estimated as the opportunity cost of capital, and thus in a range representative for an household interested in investing in energy efficiency.Most of the alternatives analyzed are not feasible: the discounted amount of savings is not enough to fairly compensate or fully reimburse the investment. Additional simulations were performed, considering several trends of the future growth in energy prices. If the energy prices grow considerably, each of the project alternatives analyzed will exhibit a higher convenience, and some of them will become feasible within the time frame considered.The results of the research can be summarized as follows: investment in upgrading the energy efficiency of residential buildings currently lacks viability, in purely financial terms. Nevertheless, it can be interpreted, as a hedge against the risk that the prices of energy supply will grow significantly in the coming years. As original contribution, this research allows to highlight a new kind of "energy efficiency paradox": improving buildings performance should allow to reduce both climate-altering emissions and, in an efficient market, the prices of energy supplies, but decreasing energy prices disincentive to invest in buildings improvement.