The most optimal way to energy-renovate an older block of flats is via external post-insulation. However, this is often not possible. Then you can instead re-insulate the internal walls, the cavities between floor partitions and the roofs. You can see a whole catalog of concrete, proven solutions here:
http://www.byggeriogenergi.dk/media/81566/katalog_etageejendomme_low.pdf
