Like all industrialised countries, Belgium accepted to diminish its greenhouse gas emissions in the frame of the Kyoto agreement. On top of the list figures CO2. A major emission source for CO2 is burning fossil fuels. As the residential sector accounts for 28% of the country's annual energy consumption and as this consumption mainly concerns fossil fuels, it has an equally important share in the CO2 release. Hence, at first sight, the best policy for a decrease is by improving the energy efficiency. The question to be solved, however, is which improvement could generate the reduction needed? This study discusses a methodology and comments simulations that help in answering that question. The results are not as simple as one should like. The housing stock in fact acts as a conservatory system. For the case being, the impact of energy efficient new construction on the CO2 release remains quite marginal if the period considered does not extend beyond a decade. The effect becomes significant only over a longer period, on condition that more stringent energy efficiency measures are combined with a shift from new construction to retrofit and reconstruction Also a diminishing increase in the number of households may help in reducing energy consumption and CO2 release. (C) 2001 Published by Elsevier Science B.V.