When there is no wind or breeze, attic vent systems work through the chimney effect, caused by gravity. Cool air has more mass per volume than hot air and tends to lift the hot air. That is, a house with 1500 square feet of attic floor space should have ten square feet of net free vent area. If the vent area is distributed equally between high and low locations, then the net free area required can drop to one square foot of net free vent area for every 300 square feet of attic floor space. Net free area means vent area unobstructed by louvers, grill material, or screening.
Manufacturers usually indicate the net free area provided by their vents.
Undisturbed air in attics will layer with the hottest layer near the top and the coolest layer near the attic floor. Ideal ventilation will not disturb this natural layering, always keeping the coolest air near the insulation on the attic floor. The cooler and dryer the air next to insulation, the better the insulation works and the less heat is conducted into the house the ideal condition. This system vents a smooth layer of air from the point the roof joins the ceiling structure of the house, along the roof deck between the roof rafters, up and out through the continuous vent at the apex of the roof. The vent air moving in a smooth sheet like this does not disturb the layers of air in the attic but takes the hottest air from all over the attic and efficiently vents it to the outside.