In a great portion of Danish primary schools the mechanical ventilation systems is outdated or simply rely on opening of windows to ventilate the classrooms. This leads to high energy consumption for fans and/or ventilation heat losses and poor indoor environment, as the ventilation systems cannot provide a sufficient ventilation rate. A recent study with 750 Danish classrooms show that 56 % had CO2-concentrations over a 1000 ppm, which is the recommended limit by the Danish working environment authority and this adversely affects the performance and well being of the pupils. This paper describes a mechanical ventilation concept to lower energy consumption and improve the indoor environment, developed for refurbishment of school classrooms. The performance of the concept is investigated through computer simulations and measurements of energy consumption and indoor environment. The measurements are made at a pilot project at Vallensbæk primary school, where a system is installed in two classrooms used by the 6th grade. The system is designed with an oversized air handling unit and duct system to reduce the pressure loss and thus fan power required to operate the system. The supply air to the rooms is distributed through diffuse ceilings inlets, where the air is supplied over a large part of the ceiling area through small perforations. The flow rate to the rooms is determined by motion and CO2 sensors and controlled by a new type of flow control dampers increasing the flow rate according to the demand. The measurements and simulation results show that the indoor environment fulfils indoor environment category II in EN 15251 and the CO2-concentration is kept below 800 ppm. The heat recovery efficiency is 85 % and the SFP-value of the system is measured to 495 J/m3 at the design flow rate, lower than 1/4 of the maximum requirement in the Danish building code. This result in a yearly fan power consumption of 4.0 kWh/m2, included a primary energy factor of 2.5. The results show that the indoor environment is improved to an acceptable level with little energy use, making the concept applicable for implementation in future energy renovation project of school classrooms.