For zero and low energy buildings, high-energy efficiency ventilation is very often confused with a complex mechanical ventilation system with heat recovery. In school gymnasiums, where large volumes have to be ventilated, and where intermittent occupation is very usual, demand controlled natural ventilation has several advantages, making this technique very attractive. High stack height makes natural ventilation very efficient, limiting the necessary number and dimensions of windows. Large air volume, with high height, combined with intermittent occupation, avoids high pollutant concentrations, especially in the occupied zone, because of air stratification, with fresh air near the floor and pollutants/heat on the ceiling. High stack effect, offers free ventilation all over the year. Natural ventilation is very attractive for architects because of no ducts, no apparent mechanical components, and low cost The article shows the ventilation concept of two gymnasiums, one equipped with demand controlled / summer comfort controlled natural ventilation and the other with a hybrid ventilation system with heat recovery. The monitoring of the naturally ventilated gymnasium results shows the perfect air quality that natural ventilation offers in this type of buildings in winter and summer. Simulations comparing a fully mechanically ventilated hall with a hybrid one and with a purely naturally ventilated hall show the energy performance of the different systems. A life cycle assessment shows that controlled natural ventilation has comparable and even better performances than a heat recovery system. Zero electricity for ventilation all over the year, and no embodied energy for ventilation compensate non-recovered heat.