There are no savings to be had from covering ventilators. How Ventilation Works Gravity and wind cause ventilation. Wind creates a positive pressure on one side of the house and a slightly negative pressure on the other and can change the air in a well-ventilated attic many times in a day. Passive vents are designed to use wind pressure to produce lift, thus drawing air out of the attic. 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.