Should soffits be insulated?
Short answer. No, the soffit itself, the boxed underside of the eave, should not be stuffed with insulation, because it sits outside the home's thermal envelope and in a vented attic it is the path fresh air takes into the attic. The insulation belongs on the attic floor, pushed out over the top of the exterior wall, with a baffle holding it back from the soffit vent.
Why insulating the soffit backfires
- It blocks intake, so exhaust vents pull air from the house instead.
- Insulation in the soffit is exposed to outdoor humidity and can hold moisture against wood framing.
- It does nothing for comfort, because a typical eave soffit is not above any living space.
The right detail at the eave
- Install a baffle, also called a rafter vent or insulation chute, in each rafter bay that has an intake vent, keeping an air channel of at least 1 inch under the deck.
- Bring the baffle down to the top of the wall, with a dam that keeps blown insulation from spilling into the soffit.
- Blow or lay insulation over the top plate up to the dam, reaching full depth as close to the wall as practical.
- Air seal the top plate so warm house air does not leak into the eave.
Interior soffits are different
A dropped ceiling box over kitchen cabinets or a bulkhead hiding ducts is also called a soffit. If it opens to an unconditioned attic above, it should be sealed at the top and insulated like the rest of the ceiling.
Floors over porches and garages
When a room cantilevers over a porch, the soffit under it is really the floor of a heated room, so that one should be insulated. Learn more in roof ventilation basics and roof anatomy explained.