In dwellings there is a desire to reduce ventilation heat loss, by reducing uncontrollable air infiltration, at the same time as providing adequate indoor air quality by controllable background ventilation. In England and Wales, the background ventilation requirements in part F of the Building Regulations at present are expressed in prescriptive terms. The adopted European Standard, EN13141-1:2004, provides a means of assessing the performance of background ventilation devices, such as trickle ventilators, and gives a standard method of measuring the equivalent area of the ventilator. This paper compares the levels of background ventilation produced by background ventilators with the presently required free area of 4000 mm2, to that produced by larger vents, with an equivalent area of 4000 mm2. Two computer models of a domestic dwelling are used to predict the ventilation rates and relative humidities achieved in houses with different levels of background air tightness, when fitted with trickle ventilators of different equivalent areas. The simulations suggest that both background ventilators that have a free area of 4000mm2 and background ventilators with an equivalent area of 4000 mm2, provide an adequate ventilation rate, (that is average heating season ventilation rate of approximately 0.5 ach-1), and adequate IAQ, (including average heating season RH<70%), when fitted in a semi detached dwelling that achieves currently recommended levels of air tightness of 10 m3/hr.m2 at 50 Pa, and in tight dwellings with levels of air tightness of 5 m3/hr.m2 at 50 Pa.