Energy retrofitting of existing buildings in Spain is crucial for saving energy. The researchers analyzed different types of housing stock built between 1980-2007 to estimate energy consumption and CO2 emissions. They found that retrofitting the building envelope can reduce energy consumption and emissions by up to 58%, depending on the climate zone. Single-family homes respond better to energy improvements compared to multi-family homes. Overall, improving the energy efficiency of buildings can lead to significant energy savings and reduced CO2 emissions.