How should attic ventilation work in Cape Cod style houses?
Why Cape Cods are tricky
The upstairs rooms sit under the roof, so the sloped ceilings are built right against the rafters. The only space for airflow in those bays is the gap between the insulation and the roof deck, often just a few inches. Dormers interrupt many of those bays entirely. The result is hot upstairs rooms in a Georgia summer and moisture problems where air cannot move.
The vented approach
- Install continuous soffit intake.
- Keep a clear channel in every sloped rafter bay with baffles, at least the 1 inch air space the code requires, from the knee wall attic up to the top attic.
- Exhaust at the ridge, with no gable vents or box vents competing.
- Air seal the knee walls and the floor of the knee wall attics, which are major leak paths.
- Insulate the knee walls and the slopes as deeply as the cavities allow.
The unvented approach
When the rafter bays are too shallow or blocked by dormers, many homeowners choose spray foam applied directly to the underside of the roof deck, turning the whole roof into part of the conditioned space. It has to meet the unvented attic provisions in IRC R806.5, and the vents are closed, not left open.
Where roofing comes in
A reroof is the best time to fix this, because the ridge can be cut for a vent and intake vents can be added at the roof edge where there are no soffits. See roof ventilation basics and attic heat and shingles.