To study the performance of different solutions for retrofit of classroom ventilation, an intervention study was carried out in an elementary school located in a temperate climate north of Copenhagen, Denmark. Three classrooms were retrofitted with four different ventilation solutions, which were:

- a decentralized, mechanical ventilation system
- a system allowing automatic window opening
- a system allowing automatic window opening assisted by an exhaust fan
- a system allowing automatic window opening supported by heat recovery units

In a separate classroom that was not retrofitted with a dedicated ventilation system, the intervention was to let teachers and pupils use a device providing visual feedback on the CO₂ concentration in the classroom, informing them when the windows should be opened. All solutions were compared with a reference classroom in which pupils and teachers manually had to open windows, as was the only means of ventilation in all the studied classrooms prior to any intervention.