Do I need soffit vents if I have roof vents?
Short answer. Yes. Roof vents such as box vents and turbines are exhaust, and without low intake they either pull air in through other roof vents or draw conditioned air out of your house. Soffit vents, or edge intake vents where there is no soffit, supply the low air that makes roof vents work.
What happens without intake
- Vents trade air with each other: on a windy day, box vents on the windward side pull air in and leeward vents push it out. Air circulates near the top of the attic while the lower deck stays hot and damp.
- The house becomes the intake: rising hot air draws replacement air up through ceiling gaps, can lights and the attic hatch. That is cooled, humid indoor air, which adds moisture to the attic and raises your power bill.
- Turbines and fans struggle: a turbine or powered fan with no intake creates suction that the house fills.
How much intake
For a balanced system, intake should at least equal exhaust. On the 1/300 path for a 1,500 square foot attic, that is about 360 square inches of intake to go with 360 of exhaust.
Adding intake
- Existing wood soffit: cut in rectangular undereave vents or a continuous strip vent between rafters.
- Vinyl or aluminum soffit: swap solid panels for vented ones and cut openings in any wood behind them.
- Little or no soffit: an intake vent installed at the roof edge above the drip edge, usually during a reroof.
- In the attic: add baffles so insulation does not choke the new intake.
Once intake is solved, consider whether box vents should stay or whether a ridge vent would serve better. See ridge vent vs box vents and roof ventilation basics.