Adequate energy efficiency in any residential building calls for a number of factors to be taken into account as specified in the energy certification of buildings under European Union Directive 2002/91/EU. In particular, the heating systems are essential to optimize the use of energy, that are both efficient and environmentally sustainable, which generally imply the use of renewable energy sources. This paper examines the impact of using biomass boilers on the energy rating and CO2 emissions in six cities located in the Iberian Peninsula with different climatic conditions. The study compares the use of fossil fuels (natural gas and gasoil) and a renewable energy source (biomass) in heating and hot water systems in two types of residential buildings. The results underline the influence of the climate in reducing CO2 emissions and economic costs, and improving the energy rating. A remarkable decline of up to approximate to 95% in CO2 emissions may be achieved with the use of biomass, as compared to fossil fuels, with the economic savings being as much as approximate to 88%. It is concluded that the use of biomass can significantly improve the energy rating in the best cases the improvement can reach four classes on a scale of seven levels. (C) 2013 Elsevier B.V. All rights reserved.