I’ve already talked about how less helmet means more money and how more vents (again, normally for more money) can mean a reduction in the aerodynamics of a helmet but there is more to a helmet’s ventilation than simply how many holes it has in it.
Obviously, the more open a helmet is, and the larger the vents, the more air that can flow through it. But to think that way is to take a very simplistic view. Helmet designers don’t just concern themselves with the exterior. Internally, good helmet designs will have channels cuts into the polystyrene that are there to direct air flow through the helmet and over the rider’s head; the constant flow taking excess heat away.
One manufacturer that has taken ventilation to its extreme is Specialized with many of its helmet designs, including the Prevail, featuring what can best be described as an open mouth at the front which directs air directly on to the rider’s brow.