There has been increased focus on the energy use in the buildings as it contributes up to 50% of global energy consumption in the developed countries. The importance of airtightness test in buildings has long been recognised due to the potentially large energy savings associated with good envelope airtightness. Moreover, good envelope airtightness makes it easy to control the indoor environment and achieve effective and efficient ventilation. Therefore, building airtightness should be used as an important indicator of building energy performance and indoor environment quality in the process of building energy retrofitting. The Pulse technique has been used to monitor the airtightness level of a range of European dwellings that went through energy retrofitting. About 10%-50% improvement on envelope airtightness has been demonstrated in most of the dwellings with one exception where the house became leakier. Lessons were learnt on issues that are potentially present in the process of house energy retrofitting.