Which parts of the attic does a ridge vent actually ventilate?
Short answer. A ridge vent ventilates every rafter bay that has a clear path from soffit intake up to the slot at the peak, sweeping the underside of the deck along the way. It does little for attic pockets that are cut off from the ridge, such as spaces behind fire walls, side attics in bonus rooms, areas over dormers, and hip corners where the rafters never reach the ridge.
Where a ridge vent works best
- Simple gable roofs with one open attic, where every bay runs from eave to ridge.
- The upper half of the deck, where heat and moisture collect.
- Attics with continuous soffit intake on both sides.
The dead zones
- Hip corners: jack rafters at the hips end at the hip rafter, not the ridge. Air entering those soffits still rises into the main attic, but the corners can run warmer if intake there is blocked.
- Separate pockets: an attic over a garage behind a fire wall, or the space over a sunroom, has no connection to the main ridge vent. It needs its own vents.
- Bonus room side attics: knee wall attics may connect to the soffits but not to the ridge, especially when the room's sloped ceiling blocks the bays.
- Dormers and small gables: their roofs may form small enclosed spaces with no path to the main ridge.
- Vaulted ceilings: rafter bays with drywall attached need vent channels; without them, the ridge slot above vents nothing.
What you should see from inside
Daylight along the peak, clean and dry decking near the ridge, and air movement you can feel at the slot on a breezy day. Dark staining in one pocket of an otherwise clean attic points to a dead zone.