Why is it bad to have off-ridge vents with a ridge vent?
How the short circuit works
Exhaust vents work by creating low pressure, and air flows into the attic through the easiest path to replace what leaves. The soffits are far below and often partly blocked by insulation; a box vent a few feet from the ridge is right there. So the ridge vent and the box vent end up trading outside air near the peak, and the air that is supposed to sweep up from the eaves barely moves. In Georgia heat that leaves the lower half of the deck hotter and damper than it should be.
Rain and pests
When a box vent is acting as an intake, it draws in whatever is outside it, including wind-driven rain and insects, through openings designed to shed weather only while air flows outward. Some leaks that look like flashing problems near box vents are really this effect.
What to do
- Remove off-ridge exhaust vents when a ridge vent is installed, and deck and shingle over the holes.
- Close gable vents from the inside if a ridge vent is present.
- Disconnect and remove powered attic fans on attics with ridge vents.
- Make sure the soffit intake is open and at least equal to the ridge vent's exhaust.
Exceptions
Separate attic spaces, such as a garage attic walled off from the house attic, can use a different exhaust type because they do not share air. Some manufacturers also make low-profile intake vents meant for the lower part of the roof; those are intake, not off-ridge exhaust, and follow different rules. See ridge vent vs box vents and roof ventilation basics.