Abstract For plant production in a greenhouse, it is very important to be able to control the greenhouse microclimate under all circumstances. In hot summer conditions, it is a problem to control the air temperature in the plant zone. The vents are fully open and the air temperature is too high. The air exchange in the plant zone is very little. Especially in modern greenhouses with roof vents only and no side wall vents, it is a perceptible problem. The air passes through the vents in the top of the greenhouse without effect on the air in the plant zone. By means of a 1 m high vertical screen placed at the top of the greenhouse, parallel to the ridge, improvements were achieved in the air exchange in the plant zone of about 50% on average. The average air temperature was reduced by 2·1°C.