THERMAL EFFECT
An effective roof/attic ventilation system uses thermal effect, the property of warm air to rise and cool air to fall, which creates a natural circulation of air in an insulated attic. Intake vents are lower, in the eaves soffit, while exhaust vents are higher, in the ridge. This speeds air movement, wicking moisture from the attic in winter and expelling excess heat during the summer.
Today, builders routinely install ridge vents; for old houses, it’s often a retrofit during reroofing.