Energy-efficient controlled natural ventilation in non-residential buildings – Experimental investigation of opening durations. In Germany, the Building Energy Act (GEG) regulates the limit values for the energy requirements of new buildings and renovations with the aim of achieving an almost climate-neutral building stock by 2050. Requirements for heat transfer coefficients, thermal bridges and air tightness are explicitly stated in the GEG, but no explicit regulations have been made for the planning of ventilation. A planned ventilation system is permitted as long as the resulting primary energy demand of the building meets the requirements. In the simplified verifications of the monthly balance procedure, natural ventilation is usually considered to be disadvantageous in terms of energy, as it leads to ventilation heat losses. Mechanical ventilation systems, on the other hand, can be considered advantageous because of their heat recovery. Natural ventilation, however, has the potential to increase energy efficiency with high user comfort if it is designed as a controlled electrically operated window opening. In a simulation with EnergyPlus, opening and closing signals of a CO₂-controlled natural window ventilation were compared for an office room. A window that opens at 900 ppm CO₂ concentration and closes at 800 ppm represented the most energy-efficient and thermally comfortable control. For this control, the real opening times were evaluated for an experimentally office room. Depending on the wind and the temperature difference between inside and outside, opening times of 3 to 5 minutes were obtained.