In an unventilated attic, the temperature will reach 150 degrees or more, when the outside temperature is in the 90 degrees range. There are several methods to ventilate an attic. One of these forces is called the thermal effect. The thermal effect results in a difference between air temperature in the attic and that on the outside, combined with the difference of elevation between the lowest and highest vent opening. In other words, hot air rises and collects at the peak of the roof.
Thermal effect is utilized primarily at low wind speed.
The other force is wind pressure. The wind flowing over the building will create areas of vacuum in the building which will result in the negative pressure areas as it passes the downwind side. Ventilation air moves into the attic through vents in a positive pressure area and exhausts through the vent opening located in negative pressure areas. The ventilation system will consist of a shingle-over ridge running the total length of the ridge. It will be made of polypropylene and will provide 18 square inches per lineal foot of net free ventilation.