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The energy efficient retrofit concept is based on a reduction of the primary energy need to a third of the German requirements for new buildings and on the environmental friendly supply of the remaining need including the neutralisation of the CO2 emissions. The main goal is to have an CO2 emission neutral building. High performance windows, an integrated insulation system with good airtightness and nearly no thermal bridges, mechanical ventilation with high heat recovery rates, user-dependent lighting and heating are measures to reduce the needs.