The office building belonging to the Remscheider Entsorgungsbetriebe REB (Waste DisposalUnit in Remscheid), which was constructed in 1968, was thoroughly renovated in 2004. Theutilisation quality was greatly improved with a combination of measures including efficientthermal insulation and solar control, fan-controlled ventilation, better use of daylight andactive use of solar energy for domestic hot water - while the energy consumption valueswere reduced appreciably at the same time.A key aspect of the renovation was to improve comfort during summer without applyingactive air-conditioning. The concept of passive cooling by night ventilation was implementedhere. The rate of heating up in summer was decreased and overheating was prevented byreducing solar gains, buffering heat gains during working hours and cooling the building byincreasing the air change rate with cooler outdoor air during the night.Measurements demonstrate that the fan-controlled ventilation guarantees high air quality.Comfort in summer proved to be very good, even though (internal) air leakage hinderedefficient operation of the night ventilation during the first two years of operation. Airtightconstruction of the ventilated zone is of elementary importance for the construction andoperation of a night ventilation system. It became evident that not only the whole buildingenvelope must be airtight but that internal connections between different ventilation zonesalso play a role.