What is the effect of installing two different exhaust vent types on the same roof plane?

Attic ventilation depends on a deliberate pressure relationship: low intake at the soffits, high exhaust at or near the ridge, with air travelling the length of the attic between them. Install two different exhaust types on the same plane — a ridge vent plus powered or box vents, for instance — and the more effective one draws its replacement air from the path of least resistance, which is often the other exhaust opening rather than the distant soffits. The result is a short circuit: air enters the lower exhaust vent, travels a few feet and leaves at the ridge, ventilating a small area near the vents while the rest of the attic stagnates. It can also drive weather in through the vent acting as intake. The correct arrangement is one exhaust type paired with adequate intake, which is what makes the whole attic participate.

Two exhaust types on the same plane let the stronger one draw air from the weaker rather than from the soffits, short-circuiting the intended path. Air then moves between the vents while the rest of the attic stagnates.

Balanced attic ventilation pairs soffit intake with a single exhaust type near the ridge. Mixing exhaust types on the same roof plane can cause one to become an intake, short-circuiting airflow and admitting weather.

One Exhaust, Many Intakes. Two kinds of exhaust just talk to each other, and the far end of the attic never gets any air.

A powered fan combined with a ridge vent is the most commonly cited version of this, and it can also pull conditioned air from the house through ceiling leaks.

• •Adding vent areas together as though capacity were cumulative
• •Assessing exhaust without checking whether intake is adequate
• •Treating a powered fan as compatible with ridge venting