A roof ventilation system lets outside air move through the attic, just under the roof deck. Air is drawn in through vents low on the roof and let out through vents at or near the top. On the way it carries off summer heat and the moisture that rises from the house, and in winter it keeps the roof cooler, which can help prevent ice dams. A passive system needs no power to do any of it, because warm air rises.
There are several kinds of vent, and no single layout suits every attic. Continuous soffit vents paired with a ridge vent are an excellent and popular option, and passive vents like these are generally recommended over powered fans.
Intake and exhaust
Intake vents go at the eaves or in the lower part of the roof, often in the soffit, the underside of the overhang. Exhaust vents go at the ridge or high on the roof. On its way through, the air has to be able to reach every part of the attic, running from eave to ridge in each space between the rafters.
The two sides need to be matched, with slightly more intake than exhaust, and never less. The amount of venting is worked out from the size of the attic floor, and building codes set the minimum.
The vents you’ll see on a house
- Soffit vents, for intake. They’re slots or perforated panels under the eaves, either running the full length or set at intervals.
- A ridge vent, for exhaust. It sits along the peak and should run its whole length.
- Box vents, also called static vents, for exhaust. These low boxes near the top of the roof have no moving parts, and they need to be equally spaced and close enough together to vent the whole attic.
- Gable vents, the louvered openings high in the end walls of the attic, which let air pass across it.
- Turbine vents, sometimes called whirlybirds. The round heads spin in the wind and draw air out.
- Powered attic fans, where a motor pulls the air out. They can be set to run by temperature or humidity, and some are solar powered.
A powered attic fan isn’t the same thing as a whole-house fan, which pulls air through the house and pushes it into the attic. People sometimes call both of them attic fans.
Where ventilation goes wrong
Different kinds of intake vent may be needed together to vent an attic properly. Exhaust is where mixing can cause trouble. Two kinds of exhaust over one attic, such as a ridge vent with box vents or with a powered fan, can short-circuit the airflow and leave the attic only partly vented. Using different vent types together also often goes against the vent maker’s instructions.
Powered fans bring a problem of their own. A fan lowers the pressure in the attic, and that can draw air up from the rooms below through gaps in the ceiling. In summer that’s air you’ve paid to cool. In winter it’s warm, moist air, the very thing the attic needs to get rid of.
Then there are blockages. Insulation laid over the soffit vents chokes off the intake, and a skylight can block the air path in the rafter space it sits in. Where the ceiling follows the slope of the roof, as in a finished attic room, the air needs a clear channel between the insulation and the deck. Baffles set between the rafters are one way to keep it open. Vents clogged with leaves, dirt or an old nest hold the air back too.
Checking what you have
You can usually tell from the yard. A ridge vent shows as a low cap along the peak. Box vents and turbines stand up from the roof near the top, and gable vents sit in the end walls. Two kinds of exhaust on one roof over the same attic are worth pointing out the next time someone looks at the roof.
Inside the attic on a bright day, provided it’s safe to get up there, you should be able to see light coming in at a ridge vent or box vents. You may see none at the soffits, and that isn’t necessarily a problem, since the way the house was built or where the insulation sits can hide it. For the signs of damp in an attic, and where the moisture comes from, see how humidity affects roof shingles.
Vents and a new roof
When the old roof is torn off, the vents set into the roof surface, such as a ridge vent or box vents, should be replaced too. Soffit and gable vents can stay, as long as they still work after the new roof goes on. That’s also the time to sort out a mix of exhaust vents left over from earlier work.
Getting a roof replacement priced? Ask us about the vents when we come to see the roof. Call (484) 553-0213 or reach us from our contact page.