Nowadays, buildings are responsible for the 40% of energy consumption (36% of greenhouse gas emissions) in the European Union. The European Council pointed out the need to refurbish a large amount of the existing building inventory, as new buildings are related to the 1-2% of the total energy consumption. Air-infiltration and tightness of buildings are usually neglected parameters during retrofitting or building design, especially in the Southern European counterparts, where air-tightness standards are absent from the national building regulations. To this effect, this study investigates the impact of tightening existing residential envelopes, focusing on the impact to the default construction and synergies arisen between air-tightness and other interventions (i.e. thermal insulation). The study was undertaken in the Mediterranean climate conditions, examining detached houses located in Cyprus. This is the first study in national level, presenting the air-tightness characteristics of buildings as these were collected by a blower door test. In general, the outcome shows that the improvement of air-tightness primarily reduces the energy associated with winter thermal loads. Apart from that the tightness of building envelopes beneficially contributes on the performance of other energy saving measures. In particular, the reduction by thermal insulation can be enhanced up to 12%, while the synergy with a glazing system may reduce heating demand up to 7%.