According to the previously reported of pre- and post-retrofit results (approximately one year after the energy retrofit) there were some varying impacts on IAQ in the two Northern European countries (Finland and Lithuania) primary due to differences in energy efficiency and ventilation systems. In Finland, building envelopes were generally better insulated even before the retrofits, owing to previous retrofits. The most common ventilation system in Finland was mechanical exhaust, whereas in Lithuania it was natural ventilation. Follow-up measurements with same protocol were performed approximately six years after the retrofit in a limited number of previously measured dwellings (42 in Finland and 10 in Lithuania). Comparing the impacts one year after retrofits and about six years after retrofits, the impacts remained unchanged or slightly improved. In Finland, the impacts on thermal conditions, carbon dioxide (CO2) concentrations and air change rates were quite insignificant. In Lithuania, air change flows were lower after retrofits. Although CO2 concentrations, especially during night-time were lower after retrofits in Lithuania, significant high concentration peaks (above 1550 ppm) persisted in about 20% of the apartments six years after retrofits. It could be concluded that more comprehensive energy retrofit actions, which have larger energy improvement potential, also have larger impact on IAQ and this emphasizes the importance of measurement of IAQ before and after retrofits.