Ridge Vent vs Box Vent vs Turbine vs Powered Fan
Every attic needs to exhaust air, and there are four common ways to do it. They are not interchangeable, they should not be combined, and the wrong choice does more than waste money, it can actively pull conditioned air out of your house.
This is the exhaust half of the subject. The intake half matters at least as much and is covered separately in attic intake ventilation, because exhaust without intake is the single most common ventilation mistake there is.
Why an attic needs to breathe at all
Two reasons, and they apply in different seasons:
- Summer. An unvented attic under a dark roof gets extremely hot. That heat radiates down through the ceiling, drives up cooling costs, and cooks the underlayment and the underside of the shingles from below.
- Winter. Warm moist air rises from the living space into the attic. Without ventilation it condenses on the cold underside of the roof deck, wetting sheathing and insulation. In cold markets it also warms the roof unevenly, which is how ice dams form.
Ventilation is a continuous low-volume air exchange, not a fan blowing hard. That distinction explains most of what follows.
The four options
Ridge vent
A continuous vent running along the peak of the roof, covered by ridge cap shingles so it is barely visible.
Strengths: it exhausts along the entire ridge, which is exactly where hot air naturally collects, so it works with convection instead of against it. No moving parts, nothing to fail, nothing to maintain. Nearly invisible. Distributes airflow evenly across the whole attic rather than pulling from one spot.
Limits: it needs enough continuous ridge length to provide adequate area, so it suits simple gable and hip roofs better than heavily cut-up ones. It only works properly with adequate soffit intake, a ridge vent on a house with blocked soffits does very little. Baffled ridge vent is worth specifying in wind-driven rain and snow markets; unbaffled products can let weather in.
This is what we recommend on most roofs.
Box vents
Also called static vents, louvres or turtle vents. Individual square vents set into the roof surface near the ridge, several across a roof.
Strengths: no moving parts. Useful where there is not enough ridge to work with, complex rooflines, hip roofs with short ridges, dormers. Inexpensive.
Limits: each vent only draws from its immediate area, so coverage is patchy compared with a continuous ridge vent. You need a fair number of them to reach adequate area, and each one is a penetration through the roof, which means each one is a potential leak point. Visible from the ground.
Turbine vents
The spinning aluminum cylinders. Wind turns the turbine, which draws air out.
Strengths: moves more air than a static vent when the wind blows. No electricity.
Limits: it has bearings, and bearings eventually seize or squeal. A stuck turbine is just a box vent with extra height. They are conspicuous, they can be noisy, and in high wind markets they take a beating. On balance we would rather specify ridge or box vents and have nothing to fail.
Powered attic fans
An electrically driven fan, either mains powered with a thermostat or solar powered, that actively pulls air out of the attic.
Strengths: it genuinely moves a lot of air, and on a roof where passive ventilation is difficult to achieve it can be the pragmatic answer.
Limits, and these are significant: a powered fan moves air faster than the intake vents can supply it. When that happens it does not stop: it takes the air from wherever it can, which is through gaps in the ceiling, out of your air-conditioned living space. You end up paying to cool air and then paying again to blow it into the attic. It can also, in the worst case, backdraught a combustion appliance. If a powered fan is used, the intake area has to be genuinely generous, and it needs to be sized deliberately rather than fitted as a default.
The mistake almost everyone makes: mixing types
Combining a ridge vent with box vents, or a ridge vent with a powered fan, is common and it is counterproductive.
The reason is that ventilation depends on air entering low at the soffits and leaving high at the ridge. Add a second exhaust point partway down the roof and it becomes the easiest route for air to enter: so the ridge vent starts pulling air in through the box vents rather than up from the soffits. The attic space between them barely ventilates at all, and you have created a short circuit.
One exhaust type. Ridge or box or turbine, not two of them.
What we would specify by market
- Denver and Parker. Baffled ridge vent, with real attention to intake. Ice dam prevention depends on keeping the deck cold and even, and that needs balanced airflow rather than volume.
- Myrtle Beach, Charleston, Savannah, Jacksonville. Baffled ridge vent. Wind-driven rain is the risk, and an unbaffled vent or a poorly flashed box vent is a way in.
- Oklahoma City and Springfield. Ridge vent, and fewer roof penetrations is better in hail country. Every box vent is something to be dented and something to leak.
- Complex roofs anywhere. Box vents, because there is not enough continuous ridge. Accept the trade-off and place them thoughtfully.
Ventilation is specified on every roof replacement we quote, and it is worth checking it appears on any estimate you compare, see how to read a roofing estimate. If your attic runs hot or you have had condensation or ice dam problems, a free roof inspection includes looking at what is actually up there.
Attic ventilation questions
A continuous baffled ridge vent on most roofs. It exhausts along the whole peak where hot air naturally collects, has no moving parts, and is nearly invisible. Box vents are the sensible fallback on complex roofs without enough continuous ridge.
You should not. A second exhaust point partway down the roof becomes the easiest place for air to enter, so the ridge vent draws from the box vents instead of from the soffits. The attic between them barely ventilates. Use one exhaust type only.
Usually not as a default. A powered fan often moves more air than the intake can supply, so it starts pulling conditioned air out of your living space through ceiling gaps, you pay to cool air and then blow it into the attic. They can also backdraught combustion appliances. They need generous intake and deliberate sizing.
They move more air than a static vent when the wind blows, but they have bearings that eventually seize or squeal, they are conspicuous, and a stuck turbine is just a tall box vent. We would generally rather specify something with nothing to fail.
Barely. Ventilation depends on air entering low and leaving high, so a ridge vent with blocked or absent soffit intake has nowhere to draw from. Adding exhaust without addressing intake is the most common ventilation mistake made.
It is calculated from attic floor area and expressed as net free area, then split between intake and exhaust. The important principle is balance rather than maximum volume, which is why intake and exhaust need to be considered together rather than separately.