Building structure is a seismic resistant reinforced concrete frame with 25 cm thick brick block walls as frame filling. It has five stories – basement, ground floor and three floors. Exterior walls are thermally insulated with 15 and 20 cm thick rock wool panels. Flat roof is thermally insulated with 30 cm XPS. External windows are made from PVC profiles with a triple low-e coated glazing and are installed according to RAL standard. Centralised heating/cooling is provided by an underfloor system using the same pipes for both. Heat (90 %) is mostly generated by a compact heat pump (air to water) with a COP 2.8 but in extreme cases there is a backup system with boilers using natural gas (10 %). The ventilation system with heat recovery runs constantly to supply 0,5 air changes per hour for the entire volume of each apartment. Hot domestic water is primarily generated by the solar thermal collectors and if necessary, supported by gas boilers.