Brad Diepholz found the following information.
Thermal effect by it self cannot create the high volume of air movement needed for effective ventilation. That is why the influence of wind is the key element in the design of a non-powered ventilation system.
Wind is a natural flow of air. So when designing a ventilation system you want to make the wind work to your advantage.
To use the power of wind you have to understand how wind force affects ventilation. It is not the velocity of the wind by itself that causes air to move through an attic.
Instead it is the winds speed as it moves against and over a homes exterior surface. A wind driven flow of air creates areas of high and low air pressure. High pressure forces air into the attic while low pressure draws air out.
A properly designed ventilation system requires balance. That balance is achieved in two ways. 1.The airflow must be balanced between intake and exhaust vents. The net free area of intake venting should be equal to or greater than the net free area of exhaust. (to determine how much net free area a particular home requires, I will talk about the formula in a later blog)
2. Intake and exhaust vents must be positioned to create a proper high-low balance. That balance is achieved when two conditions are met:
a) Half the vent area must be high in the attic with the other half low in the attic. Without that balance the area of effective ventilation is limited to the lesser of the two vent areas. (example=id 75 percent of the venting is high and 25 percent is low, the ventilation is limited to the air moving through the lower vents)
b) The vents placed high must act as exhaust vents while the low vents act as intake vents. That placement assures a continuous flow of air moving in the desired direction.
Typed by Brad Diepholz