Air leakage through a building’s envelope affects its overall durability and energy consumption, while also influencing the indoor air quality and the comfort of its occupants. To reduce the amount of air leakage in multi-unit residential buildings undergoing deep retrofits, we performed whole building airtightness tests on three buildings before remedial work commencing to obtain baseline air leakage rates for the building and to identify any major leak paths. This consisted of the airtightness test along with diagnostic examinations using smoke pencils, theatrical fog, and thermography to identify air leakage pathways through the building envelope. We then performed post-retrofit whole building airtightness tests to verify changes in the respective air leakage rates. In one of the buildings tested, the post-retrofit examination identified several significant deficiencies that were corrected before tenants moved back in. This paper summarized results from multiple buildings that underwent deep retrofits in conjunction with pre– and post–whole building airtightness testing. In addition to the air leakage numbers, we also compared building energy consumption data before and after the retrofits.