Air infiltration through the building envelope has already been proven to have a significant energy impact in dwellings. Different studies have been carried out in Europe, but there is still a lack of knowledge in this field regarding mild climates. An experimental field study has been carried out in the Mediterranean climate area of Spain and the Canary Islands in order to assess the air permeability of the building envelope and its energy impact. A wide characterization and Blower Door tests have been performed in 225 cases in Alicante, Barcelona, Malaga, Sevilla and Las Palmas de Gran Canaria for this purpose. The obtained mean air permeability rate for the 225 studied cases was 6.56 m(3)/(h.m(2)). The influence of several variables on airtightness was statistically analysed, although only location, climate zone and window material were found to be significant. Air infiltration has an energy impact between 2.43 and 16.44 kWh/m(2).year on the heating demand and between 0.54 and 3.06 kWh/m(2).year on the cooling demand. (C) 2019 Elsevier B.V. All rights reserved.