Increased attention is being directed towards reducing energy use in buildings, and implementing energy-saving measures when renovating buildings has become of central importance. The aim of this chapter is to study the effects on heat demand of a deep renovation of a Swedish post-war, multi-family building. The studied building was renovated in 2014, and the renovation measures included thermal improvement of the climate envelope and installation of a mechanical supply and exhaust air ventilation system with heat recovery. The effect on heat demand is studied through a whole-building energy simulation, using IDA Indoor Climate and Energy. The IDA model is empirically validated with regard to its ability to predict indoor temperature and energy use. The results indicate a technical potential for a 50.3% reduction of heat demand from implemented renovation measures, but measured data indicate that actual energy use is around 15% higher than the technical potential. The reasons for this gap could be overestimated heat recovery efficiency or airing.