Short Answer
Intake ventilation allows exterior air to enter the attic, commonly through soffit vents, undereave vents, or other designed intake systems.
Detailed Explanation
Intake ventilation is the entry point for outside air into the attic. The most common type is soffit vents — perforated panels installed in the underside of the roof overhang (soffit). When wind blows or when warm air rises and exits through exhaust vents, it creates a slight negative pressure that draws cool, dry outside air in through the intake vents. This incoming air replaces the warm, moist air that's being exhausted, creating a continuous flow that keeps the attic dry and temperature-balanced. Without adequate intake, exhaust vents can't function properly — they need a source of replacement air to create the flow.
Why It Matters
Intake ventilation is the foundation of the ventilation system. Without it, exhaust ventilation can't work effectively, and the attic won't be properly ventilated regardless of how many exhaust vents are installed.