The ideal system of ventilation is one where air comes into the loft at floor level from the loft front or, from a removed board in the floor at the front and exits through an opening in the loft front at the top or preferably through an opening at the front of the ceiling and thence through ventilators in the roof.
As Mark Gordon says, "this is worth repeating".
Air comes into the loft at the front and leaves at the front, flowing from bottom to top.
Taken from Widowhood Flying
by Mark Gordon
