The ultimate solution to this problem is AirLock - an ingenious innovation that is simply brought in place between the window and the window frame and which then regulates the air exchange Airfficently and Locks out pests and keeps out the rain.
Whether for windows that open to the left or windows that open to the right, bottom hung windows or skylights - these perfect air exchange guards fit all normal window types and sizes with a maximum perimeter of 400 cm.