Insect screens or thermal screens are designed to reduce heat in summer and increase temperatures during the night, but also to reduce the number of insects entering the production area. Various designs are available and each type is designed to keep certain insects out by varying the size of the holes and thickness of the material. All insect screens will reduce the ventilation efficiency inside the greenhouse. Greenhouses with a low arch will suffer the most. If you are considering a new greenhouse, make it as high as possible for maximum ventilation efficiency (Montero & Antón, 1997). If a thermal insect screen is installed on an existing greenhouse structure, care must be taken that the temperatures inside the greenhouse is not compromised to such an extent that yield losses are incurred. The are three configurations for insect screens; on the side vents, in the roof or both. C. Kittas, N. Katsoulas et al (2006) found that micro-climate variation (homogeneity) was the best when insect screens were placed only in the roof. The highest air velocity in the greenhouse was found with insect screens in the side and roof, which also resulted in lowest temperatures throughout the greenhouse.
If you find that after installing insect screens that ventilation has decreased, you can always the surface area of the insect screen. This will allow more openings for air to go through. The disadvantage is that more of the smaller insects are able to penetrate. In most cases this is a good trade off, since ventilation determines the climate inside the greenhouse which is crucial to yield.