Buildings are responsible for a substantial portion of global energy consumption. Most of the multifamily residential buildings in central Europe built in the 20th century do not satisfy the current requirements on energy efficiency. Nationwide remedial measures are taken to improve the energy efficiency of these buildings and reduce their energy consumption. Since the impact of these measures on the indoor air quality is rarely considered, they often compromise indoor air quality due to decreased ventilation and infiltration rate. We compared the indoor air quality in a multifamily apartment building in Slovakia before and after energy renovation, during two subsequent winters. Measurements of temperature, relative humidity, concentrations of CO2, formaldehyde, NO2, and volatile organic compounds were performed during one week in January 2015 in 20 apartments in one multifamily building in Slovakia. Subjective evaluation of the indoor environment and occupant satisfaction using questionnaire has been also performed. The measurements were repeated in January 2016, after the building was energy-renovated. The renovation included thermal insulation of the façade. Natural ventilation was used in the building. Exhaust ventilation was present in bathrooms and toilets. No changes to the ventilation were done during renovation. After renovation, the ventilation rates in the apartments were significantly lower than before. Concentrations of formaldehyde, TVOC and certain individual VOCs were higher. The occupants indicated more dissatisfaction and a higher prevalence of some sick building syndrome symptoms after renovation. When residential buildings in central Europe are upgraded to more energy efficient ones, the retrofitting effort should include improved ventilation in order to ensure sufficient air exchange rates and acceptable and healthy IAQ. Without these considerations, energy reconstruction can adversely affect the quality of the indoor environment.