Trickle ventilators are identified in Approved Document F as a means of providing natural background ventilation in office buildings. Research at BRE determined their optimum open areas, and their effectiveness was assessed by airflow modelling, laboratory tests and field measurements in occupied offices. Purpose-designed trickle ventilators, sized according to these criteria and incorporated in the overall ventilation strategy of an office building, can provide the required background ventilation during the heating season without compromising thermal comfort. 8 pages.