The purpose of the research described in this paper was to demonstrate that an old Danish multi-family building built in 1896 could be retrofitted to a “nearly-zero” energy building. Three types of retrofit measures were implemented in a “test” apartment to obtain practical experiences. The first measure was the installation of two different types of interior insulation, specifically, an insulation component consisting of an aerogel-stone wool mixture or vacuum insulation panels. The second measure related to the retrofit of windows in which five measures were completed that consisted of applying a secondary frame, a sash mounted on the frame or to coupled frames. The third measure consisted of installing a decentralised mechanical ventilation system with heat recovery. The results showed that following the retrofit the building’s theoretical energy use diminished from 162.5 kWh/(m2 year) to 51.5 kWh/(m2 year), corresponding to a reduction in energy use of 68%. The theoretical energy use after retrofitting fulfilled the requirements for new buildings in Denmark. The practical experiences that were retained following the retrofit were that the ventilation system ought to be installed with low noise components, insulation materials must be sized and cut to fit on site, and that new windows were selected.