Computational fluid dynamics (CFD) was used to study the effect of greenhouse sidewall vents on buoyancy-driven natural ventilation, which is the most difficult situation for greenhouse cooling. A CFD model was validated and then used to compare roof ventilation with combined roof and sidewall vents. The effects of the distance between opposing sidewall vents and the presence of anti-insect screens were investigated and quantified. In a 20 span greenhouse with a distance of 152 m between the sidewalls, combining roof and sidewall vents gave a ventilation rate per unit ground area that was twice that with only roof vents. In a 3 span greenhouse with 22.8 m between sidewalls but with the same roof vent area per unit ground area, seven times more ventilation was obtained with combined ventilation compared to only roof ventilation. In the 7 and 20 span greenhouses with roof ventilation, 79% and 48.3%, respectively, of the cross sectional area had a temperature difference between inside and outside the greenhouse greater than 4 °C, whereas with combined ventilation, these areas were 23.4% and 36.1%. In the latter case, these hot areas were located in the centre of the houses. These results show that, with buoyancy-driven ventilation, the contribution of the sidewall vents is important even for quite large greenhouses but is more critical for greenhouses with a small number of spans. Numerical simulations with an anti-insect screen having a porosity of 25% showed that the air exchange rate with combined ventilation was reduced by 77–87%, depending on greenhouse size. These reductions were much larger than those obtained for wind-driven ventilation. It was concluded that, to maximise ventilation when wind speeds are low, combined roof and sidewall ventilation should be used. Also, large greenhouses should be relatively narrow to make better use of the sidewall vents.