How does a roof vent work, and do roof vents really work?
Inside a box vent
- The opening: a hole cut through the decking between two rafters, sized to the vent's base.
- The flange: a flat base woven into the shingles, with the upper part tucked under the course above and the lower part lying on top of the course below, so water runs over it.
- The hood: a dome or box that sheds rain while leaving open sides or louvers for air.
- Baffles and screen: internal baffles block wind driven rain; a screen keeps out insects and birds.
- Net free area: the open area after the screen and louvers, usually printed on the vent, which determines how many you need.
What makes air move through it
Hot air is lighter than cool air and rises to the top of the attic, where it escapes through the vent. That is the stack effect. Wind blowing over the roof also lowers the pressure above the vent hood, which helps pull air out. For the flow to continue, the same amount of air has to enter, ideally through soffit vents.
Turbines and powered vents
A turbine adds wind powered spinning vanes that boost airflow in a breeze. A power vent has a motor and fan controlled by a thermostat. Both work on the same principle but move more air, with more parts that can fail.
When roof vents do not work
If insulation blocks the soffits, the vent has little to pull from. If a ridge vent or another exhaust type shares the attic, the two can short circuit. And if the hood is cracked by hail, the vent becomes a leak. See ridge vent vs box vents and roof flashing explained.