How does attic ventilation connect to the soffit, and why do soffit vents often fail to work?
Short answer. Soffit vents are the attic's intake, but air only reaches the attic if every layer between the soffit and the rafter bay is open: the vented panel, any plywood behind it, the blocking over the wall, and the space above the insulation. A vented soffit panel with solid wood behind it or insulation packed against it gives the look of intake with almost no airflow.
The layers air has to pass
- The soffit panel: perforated vinyl or aluminum, a continuous strip vent, or rectangular undereave vents in wood soffit.
- The backing: when vinyl soffit is installed over an old plywood soffit, the plywood must have openings cut in it. Skipping this is one of the most common hidden ventilation failures.
- The blocking above the wall: on many roofs, wood blocks between rafters sit over the top of the exterior wall. They need vent holes or screened openings, or the air stops there.
- The baffle: a foam, cardboard or plastic chute stapled to the deck that holds insulation back and keeps at least 1 inch of space under the sheathing.
- The rafter bay: from there, air rises along the deck to the ridge.
How to check it yourself
- From outside, look up at the soffit. Perforations or vent strips should be visible along the length.
- From the attic, on a bright day, look toward the eaves. You should see daylight or the baffle openings in most bays.
- Insulation pushed against the deck at the eaves with no baffle means that bay is blocked.
Fixing a blocked path
Holes can be cut in the backing plywood by removing a few soffit panels, blocking can be drilled and screened, and baffles can be added from inside the attic when there is room to reach. During a roof replacement, some roofers also add an intake vent at the roof edge where soffits are too shallow. Read roof ventilation basics and roof anatomy explained.