The member states of the European Union must increase the energy efficiency in residential buildings within the plan to reduce the external energy dependence and energy shortage. In the present work, a standard building representative of the existing buildings in Spain built before 1980 was used. An evaluation of the energy rating of that building in each climatic zone of Spain was carried out to assess the influence of these climatic conditions on energy consumption and greenhouse gas emissions. This analysis was carried out with Cerma software according to the calculation procedure demanded by the Spanish Building Technical Code. Results show that E or F energy score was obtained for the different climatic zones and with significant differences in CO2 emissions, even for climatic zones with the same energy rating. A sensitivity study of the necessary enhancements in each climatic zone was carried out to analyze the influence of most common energy demand and thermal system improvements, and combinations of them. Also, the minimum requirements to obtain D qualification in each Spanish climatic zone were obtained. As a result, it is possible to reduce emissions in Spain by over 3.5 Mt CO2 per year. In climatic zones with initial qualification E, it is possible to reach a D label, avoiding 40% of the initial CO2 emissions and reducing primary energy consumption to about 50 kWh/m2 year (over 60% reduction). For the climatic zones with initial qualification F, over 53% of the initial CO2 emissions are avoided by reaching D label. Zones with the highest heating demand (over 230 kWh/m2 year) and very low cooling demand need to improve thermal systems by means of a heat pump. Finally, some policy measures to create lines of financial aids or subsidies for the implementation of actions to reduce energy demand in Spanish homes are proposed.