Denmark is participating in IEA EBC Annex 56 "Cost Effective Energy and Carbon Emissions Optimization in Building Renovation". This paper presents results from the apartment complex Traneparken that was chosen as a Danish case study for the project. Traneparken has undergone a comprehensive energy retrofit including new facades, new windows, additional insulation, mechanical ventilation with heat recovery and a photovoltaic installation on the roof. The measured energy consumption for heating and domestic hot water before and after renovation was 139.1 kWh/m(2)/year and 95.6 kWh/m(2)/year respectively, and thereby the project has demonstrated that the renovation resulted in significant energy savings. A questionnaire survey was carried out focusing on the tenants' overall satisfaction with the retrofitting process and the results of the retrofitting, including e.g. perceived indoor climate before and after the retrofit. In three of the flats in Traneparken measurements of ventilation conditions were performed using passive tracer gas technique together with continuous registration of the room air temperature, the relative humidity and the CO2-concentration using programmable data loggers. (C) 2016 Elsevier B.V. All rights reserved.