How does air flow through attic ventilation?

The two engines behind attic airflow

Wind. Wind blowing across a roof raises pressure on the windward side and lowers it at the ridge and on the downwind slope. A ridge vent with an external baffle uses that low pressure to pull air out of the attic whichever way the wind blows.

Stack effect. Hot air is lighter than cool air. On a Georgia summer afternoon the attic runs far hotter than the air outside, so heated air rises and escapes at the peak while cooler air is drawn in at the eaves. The greater the height between intake and exhaust, the stronger the pull.

The path air should take

1. Outside air enters through soffit vents, or an edge vent at the bottom of the roof.
2. It passes over the insulation through the gap held open by baffles at the eaves.
3. It sweeps up the underside of the decking in each rafter bay, carrying off heat and moisture.
4. It exits at the ridge vent or at box vents placed high on the slope.

What stalls the flow

• Blocked intake: blown insulation packed into the eaves, or solid soffit with no openings cut in the wood behind it.
• Short circuits: a gable vent or power fan near a ridge vent pulls air from the closest opening instead of the soffits, so the lower deck stays stagnant.
• Leaky ceilings: when exhaust outruns intake, the attic draws conditioned, humid indoor air up through can lights, bath fans and the attic hatch.

The takeaway is that airflow is a system. Adding more exhaust to an attic with starved intake rarely helps and can make things worse. See roof ventilation basics for the sizing rules and attic heat and shingles for what poor airflow does to a roof.