How does attic ventilation work?

Force one: the stack effect

Warm air is less dense than cool air, so it rises. In an attic, the sun heats the decking, the air near it warms and climbs toward the peak, and it escapes through the exhaust vents. That creates a slight suction at the bottom of the attic that pulls cooler outside air in through the soffits. The greater the height between intake and exhaust, and the bigger the temperature difference, the stronger the flow.

Force two: wind

Wind hitting a house creates higher pressure on the windward side and lower pressure on the leeward side and over the ridge. A baffled ridge vent is shaped so wind passing over it creates low pressure that draws air out. On a breezy day, wind often moves more air through an attic than the stack effect does.

What it accomplishes

• Summer: carries away some of the heat radiating from the decking, reducing heat stress on shingles.
• Winter: removes moisture that leaks up from the house before it condenses on the cold deck.
• Year round: keeps the attic closer to outdoor conditions, which is what a vented attic is designed for.

What it cannot do

It cannot make an attic cooler than the outside air, and on a still, humid Georgia day it moves less air. It also cannot overcome major air leaks from the house or a bath fan dumping steam into the attic. Those need air sealing and proper ducting.

The design rules that follow

Put intake as low and exhaust as high as possible, balance their areas, and use one exhaust type so the air path stays bottom to top. More in roof ventilation basics and heat, humidity, and roof life.